Shortest patrol I've survived was 4 days. Found a large convoy 2 days out from Lorient and used all my torps on repeated attacks that night then went home.
Longest patrol excluding a milchcow stoppover was 7 weeks. Wen't over to the US coast and cruised up and down at slow ahead ~7kts the whole time. Saw and sunk almost SFA except a few lone small merchies.