What's a reasonable time frame?

The question that has plagued man for centuries (or certainly the last one) is: how  many dates before you have sex?

Over time I have subscribed to various limits myself, but largely it's dependent on a couple key variables. The first being the real interest in the woman. Girls I have been genuinely interested in being in a relationship, well, I let that take it's own path. I want to learn about the person. But, there is a limit to how long I will wait. For me, it's about time and what it is we're doing and have done together. If there isn't sex within a reasonable time frame, I'm done. The other variable is my current need. Sometimes I just need sex. I don't want to waste a lot of time. I actually become a different person when I'm in that mode.

I suppose to answer the question, I would say a reasonable time frame is one that I feel I am still getting something from the interaction. As long as a woman can hold my interest, the longer I'll stick around and wait. But she better be worth it!