If a particular item or a credit has an expiration set, this comes into affect once the promotion has been redeemed in the app or the code has been entered in the app. Basically, once the item is on the customer's saved offers or the credit is on their profile the clock is ticking.
The actual expiration works in hours, so for a promotion configured to award the customer $10 credit with an expiration of 1 month, the credit will expire (or disappear) from the customer's profile in exactly 720 hours.