Turns Owned
Each world has a "turns owned" counter. When you (or any player) capture a world, the "turns owned" number is set to 1. Each turn that you continue to own the world, the counter increases by 1, until it reaches 7. After the 7th turn without a change of ownership, the world's mines increase by 1, and the "turns owned" counter resets to 1, restarting the process.
While a world is recovering from a plunder, its "turns owned" counter does not increase.