[UKönig]

Reading "Iron Coffins", Herbert Werner tells a story of many days and hardly any days off in port.
In the early war, boats could be off line and the crew at R&R for 21-28 days. Those with technical skills would be kept around to help overhaul the boat for its next patrol, but even they would get time off.
But by the time 1943-44 rolled around, well, parts and the trains to move them were in short supply, so it was repair what you can with available materials and then back to sea as fast as possible (even if the boat was barely capable).
What used to be a 28 day stay in port with the boat lovingly maintained, was now 5-10 days and chose your top 5 repairs (and be done quickly).
Made no sense to keep boats lounging around in port when you have no idea if or when the next trainload of repair parts is coming in. Might as well do something useful for the reich, get back to sea and sink something.
More often than not, it was the u-boat that never made it. By 1944, only 2 out of every 10 boats that went out came back. Depth charges were the least of their problems.