Lüth (U-181) reported to have 200 cbm fuel left for the return trip from
well east of the Cape to Bordeaux, France = more than 5800 miles
Full tank 605 cbm, so a total of more than 17400 miles possible. Or 26 weeks....
Speed not mentioned.
Source:
Jordan Vause: U-Boat Ace, The Story of Wolfgang Lüth
Btw. found a sea distance calculator:
http://e-ships.net/dist.htm