About 5 seconds worth.
Seriously, I asked myself if I could continue living as a woman for the rest of my life?  The answer was a resounding NO, I'd rather be dead. So T and top surgery seemed far less drastic than death.
My mother asked me, what if you're making a mistake?  I said I'd already lived half my life as the wrong gender; if being male wasn't right either, then really, what was the difference?
I'm 1 year into my transition; 8 months on T, with hysto and top surgery, and I have zero regrets.  Well, I take that back.  Looking through photos recently with my kids, if there were a way to have been 100% myself as the female I was born, that would have been nice; but since I wasn't born female despite my body, the incongruency grated for far too long.
Jay