[Jimbuna]

This is a an article on what you are asking, in it the approximation is 15 to 18 rounds per minute but much would depend on the training/efficiency of the gun crew and how stable the firing platform was:

http://www.uboataces.com/weapon-deck-gun.shtml

There were two standard deck guns during World War II; the 8.8cm (on Type VII) and the 10.5cm (on Type IX). The U-boat however, was a poor gun platform since it rolled a lot, and ocean waves frequently washed over, making the gun platform slippery and hazardous. To prevent the crews from being washed over, they were fastened with life lines. A further factor was deck guns had no range finders, so engagements had to be done at close range. Depending on sea and weather conditions, it was also not possible man the deck gun at all times. The deck gun also contributed much to hydrodynamic resistance, slowing the underwater speed and increasing crash dive time. Indeed, deck gun engagements made the U-boat very vulnerable; since the gun and ammunition had to be secured and the crew had to get below deck, all of which meant that it took much longer than usual to submerge.
Three men operated the deck gun – gunner, layer and loader, usually under the supervision of the second watch officer. A chain of men were required to bring the ammunition from below the control room floor, then up the conning tower and onto the upper deck. A small watertight locker placed near the gun held a few more rounds ready for use, providing an advantage during the first few vital seconds of engagement. The rate of fire with a good crew was 15 to 18 rounds per minute.