Not necessarily, sometimes they lose interest once the deed is done anyway. It's not good to be too easy, if anything is easily obtained you'll value it less, that's just human nature. But really it's not all about time, you just have to choose the guy carefully if you're after something a bit more serious.
No. If they truly like you, having sex wouldn't change a thing but may even make them want you more. However there are guys who will pretend to like you to get in your pants. There are guys who are in between. The difficult thing is to separate them out.