my best guess would be ; it depends on how much time the gun spends outdoors.
I have guns that I've used Birchwood Casey's Tru Oil on, and they see a lot of rain and/or humidity during hunting season. So usually I reapply once every year or two. For stocks that only see fair weather (summer plinkers), much longer, three to five years or longer. again that's BC's Tru Oil, I have never used Tung Oil, you might inquire in the C&R forum. Good luck!