Once you purchase the licenses, it stays with you and does not expire. However, if you do not keep your UCSS current and smartnet subscriptions, you will not be able to upgrade to version 10 or higher. (after three years). If you let it expire, Cisco usually has a program in place to allow the customer "get well" or "get current" for subscriptions. Instead of buying new licenses, you will be required to buy upgrades or renew your subscription at a higher price.
In a nutshell, stay current on your UCSS and SmartNet contracts and you will be fine.