2-4 weeks I'd say, but as I'm on my first patrol in a Type VII, it might turn out to be longer than that! My patrols in the Type II were between a two days and two weeks depending on free time and how many events happened in the patrol.
I run at 64 or 128 TC mostly, and tend to play in real-time for a few hours when I'm chasing or attacking a ship or convoy.
One patrol took over a year, because I had a rather long break in the middle!
