The session might result in a whole life issue being handled terminatedly. If it does - great. However, that might sometimes be too big a chunk. If the person has had a lifelong issue, or if she is asking to change her personality traits - that might well take several or many sessions.
Often people will have one or two really major issues that their whole life seems to be centered around overcoming. Some real big lessons they are working on learning. It might even be their life's purpose to overcome those issues. In that case you would not expect an instant result. You might get a quick result, but more likely it will be an ongoing thing.
It is not really that certain problems are harder to handle than others. But it depends on how much intention the person has invested in the issue. If her whole life is about learning to overcome it, she won't allow you to resolve it in 5 minutes. But for another person the exact same issue might take just a 5 minute process to resolve.
Anything that "comes up" during a given week, that bothers or blocks the person, is fairly quick to handle. One session will generally always handle the "issue of the week".
If the client comes in and says "I had this fight with my wife about who should do the dishes", or "I had this car accident", or "What I heard on the news really scared me", or even "My uncle died", it will probably be handled in one session. If it has touched on something deeper it might take more, of course. But a limited event usually has a fairly fast handling.
If a new client comes and says "I've had migraine headaches for the past 20 years", or "I am shy, I am afraid of people", or "I've seen a psychiatrist for 10 years for depression, and I am still depressed", then you might count on the resolution taking a number of sessions. Sometimes you will succeed in handling it in half an hour, but don't be disappointed if you don't. But depression or headaches are not likely to be the person's purpose in life, so they are certainly resolvable. Depression is actually quite easy, as long as the person has some concept of how it would be to not be depressed.
But if somebody has organized her whole life around overcoming limitations of her race or upbringing or religion, she might not give them up so willingly. Or, if she has a high goal she is working on, she might not be willingly to look lightly on anything that is stopping it.
Also, issues that serve the person somehow can be very sticky. Many people hold on very tightly to being a "recovering alcoholic", an "incest survivor", a "rape victim", a "co-dependent" and so forth. Organizations like Alcoholics Anonymous will tend to cater to that and re-enforce it. They will typically replace a person's addiction or trauma with a just as obsessive clinging to the opposite. That might take many sessions to change into a more fluid way of dealing with it.
So, what you are handling in one session is not necessarily a whole big life issue. You don't have to worry about maybe not changing the whole person miraculously. What you have to concentrate on is the finishing of one major action at a time. That major action might correspond to one life issue, or to a small portion of one.
If you know your tools well, you can control the length of one session quite precisely. If is up to your skill, not up to the client, how long time it will take.
I frequently do sessions in a doctor's office, where patients are scheduled to see me every hour. That generally works quite well. I would tell them that sessions, in principle, are 50 minutes and that it might be a bit more or less. Telling the client that there is a certain expectancy of duration makes it easier to control.
My actual sessions are typically between 40 minutes and one and a half hours.
My clients will generally know when the sessions is done, simply by the amount of change we have gone through. It is not uncommon that we have done enough in 30-40 minutes so that the client simply can't handle any more, and will say so. They will just need to go out and use it.
If we are working on a complex structure, it might take longer than an hour to get somewhere. For example, we might spend 20 minutes trying to find out what really is going on, and we might find that it is a polarity. We spend 30 minutes working on it with polarity integration and we might find that an unwanted feeling is keeping it stuck. We spend 30 minutes doing re-experiencing and handle the feeling. And then 10 minutes to wrap up the polarity integration. So that could easily be 1.5 hours, even if you work very efficiently.
You don't want to leave something in a stuck state. You have to get it to some point of release. Even if it is not complete, you would like to leave it in a harmless state. The client has to feel that we made progress and that it is safe to put her attention out on other things again.
Where you have some leeway is that, once you have made some progress, found out what we are dealing with, and resolved it somewhat, then it is pretty much up to you when you want to pull out of it. You can do a very thorough job and finish it all in this session, or you can do a quick job, get her to validate a gain, re-establish her solid grounding, and let her come back for more some other time.
If you are doing re-experiencing, you might go through a number of incidents and resolve a whole unwanted feeling in one session. Or you might do just one incident to resolution of that incident, but leaving the rest of the feeling there. The key point is that you are able to get the client's attention off of the subject matter and back out on the external world, with a positive outlook. If you can do that, it is generally safe to end off. If she still has her attention fixated on the problem and she isn't happy about it, then you can't end a session.
If everything else fails, the session has run for hours, and you don't know what to do, then you would have to resort to more crude and cheap tricks. You still can't end the session with the client buried deep in misery. But you can use whatever you can think of to get her attention outward, thinking about something else, having some space. Tell her a joke, start a discussion about things in the room, take her for a run around the building, make a sudden change in activity. That doesn't do much for the unresolved issue, but it might get her into a state so that you can end the session and let her loose. And then you can work on figuring out what to do before she comes back the next time.
Just continuing the same action is not necessarily the solution if it ain't working. If you really don't know what to do, and it is just grinding on and on, then get the session ended any way you can.
There are a number of techniques you can use towards the end of a session to put the client into a suitable state for ending. The general rule is that you shouldn't start using them before you have gotten some stuff resolved. You find something to work on, you apply some techniques to it, you get improvement or resolution, and the client is aware of it. Then, when it is time to end the session, you can bring in extroverted, grounding types of techniques.
Simply having the client look around the room might be enough. Or talking about how her future will be different based on what she got out of the process. That puts her attention outward, towards a positive future. That is a very good way of leaving her.
There are various imagination processes that can be applied in the latter part of the session. Visualizing a positive self-image, making it bigger and better, copying it, spreading it around.
If you plan on doing both negative processing (getting rid of something unwanted) and positive processing (putting something desirable there) in one session, do the negative stuff first and then the positive stuff. After dealing with all the negative stuff, her sense of abundance might be going down and she might still have a bit of attention on it. Positive processing is the perfect way of remedying that.
The way you structure the session will allow you to control the session length. If you don't start more negative stuff than you think you can resolve, and you use positive techniques to complete the session, then you can be quite precise about how long a session is.
Don't promise the client or anybody else that session timing is precise. Tell them that the target is 50 minutes, or 1 hour, or 1.5 hour, or whatever you choose to be the norm. Try to hit the target, but don't worry too much if you don't.
If you are seeing several clients after each other, leave enough buffer zone in-between. Time enough for sessions to run over, for people to get in and out, and for you to write up session notes and get a break. Until you are confident that you can be more precise at it, you probably shouldn't schedule people closer than 2 hours apart.