The good news is that the Telerik controls do not require license keys and they do not expire. And there's no bad news :).
Once your subscription (usually the subscription is for 1 year) expires you may decide to renew it in order to get access to the versions which will be released during the next year but you may as well decide to stay with the latest versions you've acquired.
Whatever your decision is, you should know that you don't need to update anything (license keys or dlls) in order to keep your older versions running.
Should you have some other questions, don't hesitate to contact us.
All the best,
the Telerik team