Quite frankly, it could take for what seems like forever. They aren't required to answer in any given time frame, and likely will wait to see how many offers than can get in before responding. That said, many companies holding mortgages these days are trying to clear through their inventory fast because of the number of foreclosures they are having to deal with. The more foreclosures they have on records, the less they can lend out on new mortgages.
You can risk putting a time constraint on the offer, but you better be able to stick to your guns and move on when the time frame passes by. Since there are so many foreclosures these days, you certainly should have a lot of other options to look at if this particular home doesn't pan out.
But patience is a big virtue with foreclosures.
Best of luck,