I think for many issues therapy is a bit shorter term. Or spans of time from one point or another. Either because of the issues, or because of the person getting as far as they'd like, or just feeling better.
Some people stay in longer because of the issues they are dealing with, ability to integrate, desire, because it helps them cope on a full time basis, or they just want to and it helps for some other reason.
My T said many people worry about being terminated 'too early' and sometimes people actually will not improve as a way to keep the relationship. My T says she knows when this occurs, and brings it up and they work together to find out what is up.
I asked my T continuously when things would be done. She'd never say a timeframe… or sometimes she'd be like 'maybe a year!' but I later found out she was just throwing something out there to me. LOL because after a year I'd have all this shame and we'd talk about it again.
I think ending is a decision on the client and T's side. If a T doesn't find they are giving benefit (which I doubt many do, but I'm not sure) someone who wants to be in therapy can find someone else who CAN work with what they'd like to continue to work on.
I'm so sorry you are frightened
it sounds like just an awful place to be. I've been in a similar place.
There is no 'supposed to' - there are statistics, like anything that can be measured… but all therapy is individual.
I was just talking to my T the other day, on our 4 year therapyversary… and we talked about how far I've come, that I can make it on my own (or at least better) but that it's so wonderful I want to keep working. She also mentioned that she believes I've been through so much trauma that we're I'm at is beyond expectation. With the number of sessions I've had, I've probably received the weekly equivalent of 15-20 years of therapy. I've asked T about her long term clients also… she's had some for 10 years… though they may not come all the time. She'll certainly be gone and retired by then for me.
Anyhow… sorry to ramble so much. I hope you can bring this up with your T. It is honestly individual and shouldn't be mandated to end at a certain time IMHO.