In real life, the average tonnage per patrol for an "average" commander was a lot lower. According to uboat.net, Otto Kretschmer is credited with 46 sinkings for 273043 GRT in 16 patrols (224 days at sea). That comes out to just a little over 17000 tons per patrol. And he was the most successful commander of the war, and operated only from Sept 1939 to March 1941 when Allied defenses were relatively weak.
The game is a whole different kettle of fish, since so much depends on the realism options selected and on whether or not you choose to play with additional "realistic" restrictions that the game doesn't enforce. Some mods no doubt also have an affect on how easy it will be to locate, catch, and successfully attack targets and still get home alive.
In my current career I've got about the same total tonnage as Herr Kretschmer finished up with after 313 days at sea since the war started. But even if I played at 100% realism (I don't - more like 65-70% ATM), it could never approach the 100% realism he had to deal with, lol.
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