The term "refit" is really a misnomer. All you are doing is loading torpedoes and fuel, and maybe making a few minor repairs. This would likely take hours to a day.

At the end of a patrol, a refit crew would take over, and start repair and the crew would have shore leave (if you could call it that). After they were rested, a period of training would commence and finally some exercises before the next patrol. This would usually be about 3 weeks, unless major repairs or such were to be done. Then it could be much longer, as extensive overhauls required shipyard work in the States.

The fleetboats were designed to carry stores and fuel for 75 days, but generally were scheduled for 60 days. However, it often happened that patrols had to be terminated for various reasons, including fuel, torpedoes, mechanical problems, battle damage, illness, etc.

Personally, I feel a patrol should be limited to about 60 days, or 75 at the most. Say about half that for a S-boat. Early in the war, operations were conducted on an emergency basis, due to desperate circumstances, but that is a bit different.

As far as the "double-barrel" patrols are concerned (using "refits" to get more torpedoes and fuel), I doubt that many skippers were willing to prolong their patrols much beyond 60 or 70 days. Patrol ops were very fatiguing and stressful for the crew, and they would likely be worn to a frazzle after that length of time.