You will get new tasks as long as you have torpedoes. If you resupply in a harbour you have torpedoes again and therefore you get new tasks. You will never get an order to return to port (maybe you will at the end of the war but not in the middle of it). Just head back when you are low on fuel or torpedoes and end the patrol (I think that's the most realistic way).
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Putting Germ back into Germany. 
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