Los Angeles, CA here...
My (former) therapist is pretty well known in the trans* care world. According to her, no two people are exactly alike, so it's a case-by-case thing as far as she is concerned. In my case, I was considered full-time on the day I stopped presenting myself to the world as what would be interpreted by most as male. I told her via email one day that I had made my decision to go PermaJill, and that I had already been full time for two months. She started my "clock" backdated to then, I had a few more sessions where we seemed to mostly talk about wine and heavy metal (per WPATH guidelines). I got my full HRT letter last July and my SRS walking papers in March.
She would not have done that for everyone, mind you. I don't have an employer, and it seems I will never need one. My wife is very successful and our families are both very well off these days. At first I didn't think a full transition would be for me, and I was hesitant to even try estrogen (big dummy moment). After all, I had convinced myself I was actually non-binary and I was truly afraid of the unknown. At one early session, she told me that I was actually the PERFECT candidate to transition successfully, and that most transpeople would want to smack me upside the head for not doing it yesterday. After all, I am built just like Julia Child... Had I been disowned, divorcing, unemployed, extremely unpassable, had kids and/or struggled financially, she would have been hesitant to recommend transitioning until I could do so successfully and keep me on a therapeutic dose of estrogen until that time. In her experience, unsuccessful transitions too often end in preventable tragedies, and this is what therapists must avoid at all costs.
The answer is that it depends on your situation and your therapist's take on it. Some are more likely to give you the letter ASAP, some want to cover their own asses first, and some are gatekeepers who see a cash cow every time a transperson walks in. If you run across one who wont even give you HRT until you are full time for a year, it's time to run screaming.