Very first time? About 4 years and she turned me down.
Second time was on the first day I met her, she said "yes", and since then I usually asked girls out pretty early (usually within weeks of meeting them, and not so rarely even on the first day after I was introduced to them).
Mostly I got out of the "crushing" phase, never allowed myself to develop such things. I'd ask a girl out if I liked her even a little bit, and got over the fear of being rejected.