All commercial CKEditor licenses (CKSource Closed Distribution License (CDL)) are lifetime and perpetual. Your right to use and distribute the purchased software never expires.
Each purchase also gives you the right to upgrade CKEditor to all newer versions released within a year after you buy a license as well as to use the dedicated support channel during this time. After a year from the date of your purchase your access to the support channel is terminated and you can no longer upgrade CKEditor to versions released after this time.
Each license can be renewed yearly at a discounted price in order to make it possible for you to take advantage of latest editor versions and maintain access to professional assistance from the CKEditor development team. To renew your comercial license, just go the the CKEditor Premium website and scroll to the Renew CKEditor Premium support & upgrades section.
The full text of the CKSource Closed Distribution License is available here.