These are opinion questions...so don't be expecting one to be right or one to be wrong...

one of my Ts said 12 - 20 sessions are good for specific issues such as how to deal with a family issue (ie... grief, new person in family, etc..) perhaps even phobias... or even anxiety...

but that same T says lots of people come in for what seems like one of these issues and its just the beginning of figuring out stuff... or the true issue turns out to be trust issues, or PTSD or etc, etc. etc and you start to look at the causes and bamm you are into therapy for a couple of years...

then that same T says some people have a mental illness that will be with them forever even if better controlled by medication that may need to have a therapist for the rest of their life...

I took that to mean there is no right answer for everyone.