I was accepted off the waitlist at NWP. As I recall, it took a few weeks, but I'm really not sure. The point is, it happens.
As far as the decision, you need to to do right by you. Take your time. It's good of you to want to open up space/put people out of their misery, BUT you never know what you'll find out once you are accepted somewhere. Plus - and this is key - once you have an offer, you know what questions to ask another place before you accept. And of course, you may have new questions for your first offer.
What's in the brackets are your thoughts, not necessarily things you want to put into your queries:
[I've received an offer from a school that guarantees housing,] Do you guarantee housing?
[I've received an offer from a school that provides travel stipends,] Are there opportunities to attend conferences and participate in off campus events?
[I've received an offer from a school that has very strict residency requirements,] How strict are your residency requirements?
etc.