If you buy Fences now, your license will be for Fences version 3.XX. If Fences 4.XX comes out, you will have to buy another license for that version - but you'll still be able to use your old license with Fences 3.XX.

 It is a one time fee for the version you are purchasing. If a newer version comes out, as explained by Delicieuxz, then you pay another one time fee for that version. The fee is only for the version you are purchasing, and you can use that version forever.

The answer from stardock should then be something like...

We aim to release a new version every X years, and maintain updates for X+Y on existing versions. Version 3 was release on Z.

Whats to stop stardock from just releasing a new major version every 6 months? Please point us in the direction of the release roadmap or something...

I own fences 3.x, but I'd prefer to have access to all older versions, not just the latest... an older version may not have all the features, but I'd rather something that works.