Depends a lot on the nature of the contract, but not long - generally if I'm hiring a contractor it's because I have work that needs to be done real soon now. Unless you're prepared to commit a decision in a day or so, then I'm going to be talking to other contractors, because I can't afford to wait three days and then hear "no sorry, I've taken another job" - and if one of those other contractors is prepared to commit to a decision, then I'll likely go with them rather than with you.
As always, communication is key here. Keep your potential client informed as to what you're doing - I may be prepared to wait some extra time if I know when you're going to make a decision; the worst thing of all is for radio silence when I have no idea what you're doing.