Wonderful to hear of another supportive partner, Hayden!
Ultimately, the decision for the length of therapy is something you can decide with your psychologist. Some therapists want to see a client for a specific length of time; some only require a session or two; the client may want a lengthier term to have a safe space for discussion; or, may just want the HRT and/or surgery approval and be out of there.
Personally, I had one 3-hour session with a private psychologist, which included some tests, and received my HRT and surgery approval. That was all that was required, though I did end up going back to her once or twice because I wanted to.
Because there is a wait list to see an endo where I live in Canada, from 3-8 months, I had my doctor submit my referral to the endo prior to receiving my psych assessment. The endo did require that assessment to be completed before prescribing, but getting the ball rolling earlier meant an earlier HRT start date. So, I recommend scouting for an endo now.
Hope this helps!