I have seen my therapist on and off for over 4 years .... it was pretty routine in the beginning and I was probably at a point where I could have gooten the recommendation letter and started, but I wasn't mentally accepting of my condition yet and my therapist knew that.
I took some time off and did some soul searching and came to accept myself as being transgender and someone who was going to eventually go through transition. I have gone back to therapy after about a two year break ... and my therapist wants to get to re-know me and make sure I am of the right state of mind. I have since seen her twice and I have a couple more sessions before she'll write her recommedation letter. I fully understand and am accepting of this, because it is such a huge step to take in the process.
Most of the time these therapy session are going to be spread over a couple of months and usually each appointment will have a few weeks in between. Therefore it may take a several months & several sessions before your therapist is comfortable enough with knowing you to write such a letter. It's takes patience and persistence on your part, but it's also being consistent with who you are and your therapist will read that in your sessions. Just be yourself, be honest, and openly communicate about your goals and plans and you'll be fine.
Best of luck, Jennifer R.