I also always use Random Days in Base except for my first patrol. I do a quick turnaround, set up my crew the way I want them and then set Commander for 18 Days in Base. This way my first real patrol will depart on the 19th, just as the original first war patrols did. After that all the downtimes are random.
You want a short turnaround? I once came back from an empty patrol in a Type II with all five torpedoes still on board. They had me out at sea again in just five days! I loved it.