My first girlfriend, we were both virgins. We'd been dating for about a month before we went beyond kissing, about 2 1/2 before we had oral sex, and we'd been dating for 7 months when we first had sex. Dated for a little over a year after that.
Now, I've had one casual partner who I had sex with when we weren't in a relationship. We never got into a relationship.
My current girlfriend, I guess it was about 2 weeks? Maybe not even quite that. It just felt right at the time. We're coming up on 4 months now. It really depends on how experienced you both are and how close you are to each other. If either of you is a virgin or younger than about 16, you'll probably wait longer than more experienced/older people.