I thought the average patrol length was thirty days but I'm no U-boat scholar by any stretch of the imagination.
I belive the longest patrol I've had so far, out of ten patrols, is twenty-eight days. I usually expend all my torpedoes within twenty days and head for home.
It's currently Christmas Eve, 1940, and my crew and I have been on patrol for two weeks with not a single ship sighted. You couldn't cut the restless boredom on board with a chainsaw! Although, when the action does start, I'm sure every last soul in the crew would gladly exchange the inevitable "tin can fever" for the cabin fever permeating the confines of U-47 right now.