for me, the issue is somewhat mixed. i have had moments when i wanted sex... be it the first date or be it the 50th date, it didnt matter to me. i also have times when the girl is definitely interested in having sex with me, but i want to wait. so, it all depends on the mood, on what the guy really wants out of the situation, and also what he thinks of the girl.
if he thinks she is easy before he asks her out, then he will be expecting sex. if he thinks she is a prude, then he wont be. if he thinks she is normal, then he probably wont start expecting it until a week or so has gone by... but again that is all dependent on the guy. do note that a week is a "short" time period in some cases, but that depends on your age as well. in college, a week is about average. in later times, a week may be quite short. before college, a week might be very quick. it also depends on how often you see that person during that week.
personally, i dont have sex with any girl until i am in a relationship with them (at least now i dont)... and once i am in the relationship... it can happen at any time. typically i will get to know a girl for a few weeks before i think about a relationship with her. if we are in a relationship, then it is natural progression to move to sex, not necessarily right away.. but within the first few weeks typically. it all depends on where both people are in their lives and on the situations that are between them.
that make any sense? i could ramble on more...