This depends on waaaay too many factors: like who the person is, how long you've known them before you start anything official, whether or not you (or they) are a virgin, how desperate you are (and I don't mean that negatively..I mean...'keen'...), what their opinions are, how old one or both of you is/are...
My answer would be the supremely unhelpful 'long enough'. ;-P
My answer would be the supremely unhelpful 'long enough'. ;-P