I don't know how common long term therapy is. As previous posters have pointed out, it really depends on why you're going to therapy in the first place.

I think my T originally thought I was going to be short term. Then we stumbled upon my issues. I've been in therapy for 2.5 years now with the same T and I don't think that will be changing any time soon. I spent way too long feeling way too bad. I'm going to do whatever I need to feel better and that will probably include long-term therapy.