It does vary widely. I know a therapist in town who, last time I checked, was willing to write a letter after only one or two visits, even when (in my lay opinion, based on what I saw in support groups) it wasn't a very good idea.
Other therapists seem to string people along or state ridiculously high requirements.
My therapist adhered to the old three-month standard because he wanted to properly assess people and give them time to reflect. I imagine that he still thinks it's a good idea. However, I didn't start HRT until six months into therapy. He agreed to write a letter after something like four months--when I finally officially asked him--and then it took me some time to get an appointment. First I delayed out of sheer terror, then the endo's office gave me incorrect info and refused me, and then, when we got that straightened out, there was a one-month waiting list for an appointment.
Some people who know they will have three months of therapy make the endo/medical appointment before the three months are up, to save time. But that doesn't always work.
