You can recover turns at any time you wish but there is an extra cost. You have to donate 1 more time to recover at most 1,000 wasted turns. So if you wish to recover 2,000 turns, you have to donate 2 times. Donation to become premium doesn't give you a point.

Wasted turns can be used only to upgrade your robot. You can't use them to do anything else. You will normally gain HP , EN , money as normal turn when you spend wasted turns. You will see amount of wasted turn that has been recovered on the upgrade page. Wasted turn can't be used for special upgrade

If you delete you robot and create a new one, or you just create new robot today, all turn before the time you create a robot is counted as wasted turn.

The Turn Recovery function can be found in menu bar Special Actions -> Point Service.
You can recover turn only when your special turn available = 0

If you wish to recover your wasted turn, just do extra donate at any time and you'll get point to spend for Turn Recovery function within 24 hours after you make a donation. Your point will last until end of season. This means you can donate today and recover turn whenever you wish within this season.