Full time with a male name, pronouns, male title on (any non-legal) documentation, out to family, job interviews as "Mr. SomethingOrOther" - about 10 months.
Legal name change (ie - actual legal paperwork to prove I was living as male), male titles on legal documentation - About 6 months.
But I was not in therapy for that time (not until right before I got the T) 'cause I was too broke and wasn't aware of just how little it would actually cost! *Facepalm*.
I wanted to make sure I was certain in what I was doing before I started (what I thought would be) blowing a heap of money on the shrink.
My identity on the other hand is different story. Hate to play the stereotype and risk blindly parroting the "party line", but I've always had a male internal identity (even if I did try to consciously ignore it for a few years). I honestly can't remember thinking anything else. I've always dressed in male clothes and just followed my own way of doing things, and I've been using what is now my legal name in private (and online and such) for more than 10 years.
It sucks right now kiddo, but don't go doing anything rash. Trust me, it gets better from here. Waaaaaaay better

. Hang in there.
In the mean time, try making any other adjustments you can to make your life more comfortable.
Look into buying a binder (if you haven't already), and perhaps ask about being known as a different name (non-legally) at school. If you have a school counselor, drop in and have a chat with them. While they may not be able to get you T (or anything like that) they might be able to help you get called something else in class, or be allowed to wear the boys uniform (if you are unlucky enough to have uniforms), or just be a staff member that listens and is on your side.