There is no secret number on how long to wait. It should just come naturally.
Overal though the longer u make him wait the more wooped and into you he's gonna be. If he likes you it's just gonna drive him nuts and make him want to see you more. If he gets bored waiting and dumbs you then he was just after sex rather than you.
General rule of thumb....the quicker u sleep with a guy the more likely you'll just end up being fuck friends.
If you want a real relationship with him then make him wait several, several dates and then plan something romantic like a weekend getaway.
Oh wait, you're just a college girl living with roomies.....in that case go to a kegger, get wasted and fuck him wherever
j/k