For Individual license purchase, it is yours for life. It is not a subscription. So for two license it will be $20 for life. But do know that:
With the individual license, incremental upgrades (Fences 2.0, 2.1, etc) are offered with most Stardock software at no charge. Version upgrades however (Fences 2 -> Fences 3) are usually paid upgrades. (From here: https://forums.stardock.com/486310/object-desktop-support-faq#objectdesktop)
So for Windowblinds case it will be free update for V10.87 and above. And it should be a paid upgrade when Windowblinds move to V11.XX
Stardock Community Assistant.