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; and then it resets to 1, restarting the process.
When the "turns owned" counter resets in this way, after the 7th turn without a change of ownership, if the number of mines at the world is at least 1 to start with, then the mines will increase by 1.
While a world is recovering from a plunder, its "turns owned" counter does not increase.