As for how long - each (2) therapist I've seen said they were pretty sure after one visit and were definitely sure after two. I only know that because I asked. Which is kinda funny since it wasn't until after visiting the Medical Dr and describing to him what was going on and he stopped me and said "Bingo - that's Gender Dysphoria." To be honest - that made me feel 100 times better knowing that I wasn't mis-self-diagnosing and the therapists just "going along for the ride."
Note this - I learned something the other day about therapists. I should have paid more attention earlier on in the process. The letters after their name is important.
I don't really know what all these letters mean (Ed.D, LCPC, NCP, DAPA) but apparently they hold weight for insurance companies and medical doctors. But the first therapist I was seeing had these after her name: MA, LPC, ACCHT. Apparently that is not enough in the eyes of insurance at least. it also means $100/hour vs $180/hour. But as the old saying goes, you get what you pay for. Also note some therapists will adjust their pricing based upon income.
Also drug companies will help out too if you can't afford prescribed medications. Consider asking a pharmacist to see if there are any discount drug programs for the specific drugs being prescribed. Or look online at pharmaceutical web sites to find help.