I'm too lazy to find a direct answer, but you might want to take a look at
https://www.hnsa.org/manuals-d...orpedo-fire-control/
It's the Historical naval ships association page that has scanned and translated manuals from WWII era for german and american submarines.
The 1943 german submarine manual states that
quote:
The submarine as a gunnery vessel is in itself properly a contradiction in terms.
an interesting snippet from sci.military.naval
quote:
D K Brown describes his time in HMS Tabard, winning the Fleet gunnery contest: based on the tactic of surfacing close astern of the target and aiming for his stern gun, the competition started with the umpire's "Go!" at periscope depth and ended when the first shell hit the (small) target: the aim was to surface about 300 yards off the target's quarter, and strong men were picked to open the hatches as the boat surfaced: Brown estimates about a ton and a half of water would come in.
suggests it's really in the 30 second range for a prepared crew and mounting.
Also, the sub commander might want to not make the interval too short because the crew might forget to unplug the gun, with embarrassing consequences. It's happened a few times.