How do auto-expiring coupon codes work?
If you want to setup discounts that auto expire after a certain time (1 day, 2 days, 7 days, etc.), you need to set the *Days to expire discount* under the *Advanced* tab on the rule. This can only be applied in lengths of days, at this time.
The way this works is that every time an email is sent to a customer that has a discount associated to it, the customer will receive a unique coupon code. Then, after the time period has passed for each individual customer, their coupon code will be expired.
The way that Magento works out of the box in regard to coupon code expiration dates is that you can set a single expiration date for the entire shopping cart price rule - not for individual coupons.