How long can I leave money sitting in my Payoneer balance?

Leaving it there is not prohibited, but if you intend to use the service export earnings deduction, the entire earnings must reach Turkey by the end of the filing period. For a freelancer filing annually, that period is the practical limit. If part of it stays in the balance, the deduction is not reduced proportionally; it is lost completely, including the part you did bring in. This is why the transfer calendar is set before year end.