Off by a factor of two, each tube would fire a round every 30 seconds or so. If my memory is correct, the max time of flight is on the order of 90 seconds, meaning it's theoretically possible to have 3 rounds in flight from each gun.
Of course, multiply that by 9 guns and you can have 27 shells in the air heading at you at the same time. Not something I would want to be on the receiving end of!