From what I understand. If you buy the current version of Fences, the key that come with it will be valid forever for that version for one computer/device only. ie. If you buy today, you will get the key to go with Fences V 184.108.40.206 and it will be valid for all V3 including updates. But if Fences get an upgrade to Fences V 4.X.X etc, you will need to purchase a new key for it.
If you prefer you can purchase Object Desktop pack . Which include Fences and a tons of other great products. This also offer free update and upgrade. And also you can get all beta version before it get release to the public. Object Desktop pack is subscription base which has it yearly fee. More information on it here : https://forums.stardock.com/486310/object-desktop-support-faq#objectdesktop
Stardock Community Assistant.