Here's a useful page:
http://uboat.net/ops/top_patrols.htm
As you can see, some pretty impressive patrols were made in real life, although given the total number of outings, the fact that there's only 31 patrols with over 45,000t recorded - well, you can see these were not the norm. But some (like Luth on the U-181) certainly had repeated success.
Note the mention there, by the way, of the fact that most of the really successful patrols were long cruises by Type IX's in distant, poorly-defended waters.