The windows store declares that if you create a durable product with a lifetime other than forever, it will be available only for Windows Phone 8.1 users. How can I deal with Windows Phone 8, if I want a product to last only for 30 days and not forever?
I assume from your title and tags that you mean Windows Phone 8, not Windows 8. Windows 8 supports expiring durables.
Windows Phone 8 doesn't support expiring durables, but you can use a consumable and either not fulfill it for 30 days or fulfill it but not offer it again for 30 days.
--Rob