Well, it depends on the level of friendship or tension between us from the get-go. Time frames seem to be situational. I've had a wonderful 3 year relationship that started with sex the first night. I've dated one man who after 5 months still didn't want to cross that line (which ended it for me. made me realize we wanted different things in more than just that area). I do know women need to feel safe, secure, and desirable both physically and emotionally for them to consider it in the begining of a serious relationship. meaning that's not always needed for a romp in the hay, which also means women sometimes wait while they figure out what kind of relationship this is going to be. For me, a few dates to figure out if we mesh well personality wise, then I have to find out if we mesh well bedroom wise. Sex is very important to me, so I know I don't wait as long as most women, but I also am pretty picky about who I date. I think that also makes a difference in how long women wait - how important sex is to them. Hope this helps.
For me.I have to make sure that he wants to be with me and he is serious about this relationship. Since I want to be in the relationship with the guy that mean I already made up my mind of about having sex with him.
I think 3 months is a good time period because by then you should know if you are going to work out or not an are right for each other and you should know each other well enough and be comfortable but yehh :) I'm not actually a girl soo why am I answering this question :S