If you purchase DeskScapes 3 as a stand alone product, you will receive the program plus free updates until the next version number is released (DeskScapes 4). Since 3 was recently released, it will probably be a while before version 4.

The program will not expire but you should always create an archive copy to have on hand.  

It sounds like you're thinking of an ObjectDesktop subscription, which includes DeskScapes. The benefit of ObjectDesktop is (so long as you continue to renew every year) you will have access to the latest versions of Stardock applications. Note that even with standalone purchases, we typically offer the next major version at a discount to previous owners.