At what battery percentage should the battery be recharged?

Whenever you need to recharge to to insure you have enough battery use when you need it.

Just like a car, if you have half a tank of gas and only plan to do local errands for the next few days it will probably be enough and can wait to refill it. However, if you are planning a log road trip and don't want to stop for gas you fill it up then.

However, unlike a car, you can just leave the MacBook Air plugged into the charger when not taking it out and about, Once the battery charge reaches 100%, the charging function will be turned off and the MacBook Air will get its operational power from the charger allowing the battery to sit at full charge when you need it.

Apple sets the break point at 80%. But they want to sell you a service and battery as often as possible too. woodmeister50 gives an excellent response.