This time period is set by the business owner, and therefore varies from program to program. Most rewards are valid for 30 to 60 days after they've been purchased from the rewards store.
Occasionally, business owners or program managers will send special time limited offers directly to your app. Sometimes they'll only be good for that day, or thru the weekend.
The expiration date of a reward/offer (as well as additional information about the deal) is shown under "My Rewards" when you tap the rotating arrow on the right.