It seems like more and more schools — especially the early interview places — are moving towards "exploding" offers with relatively short deadlines (1-2 weeks after the official offer letter has been received).
On the other end of the spectrum…Last year a friend of mine held and offer for over a month while his wife looked for employment opportunities in the area. Ultimately he turned down the offer because there were no opportunities for his spouse. It seems like a legitimate reason for holding an offer is to find spousal work/check out the area. I would assume that schools would not appreciate you asking for more because you are waiting to hear back from another place because that implies that you like this other place more.