There may be some prescibed rate of fire in a relaxed situation. I haven't found it. But I still believe it was up to the crew and their command in a given situation. The fourth detachment of The Third Company, Richmond Howitzers (a twelve pounder Napoleon) were reported to have gotten of 11 rounds in the first minute of firing at Pole Green Church in 1864. They were under attack from infantry in column. Their other section gun was almost 100 yards away. That's coming from a diary account. With the proper loading procedure and relaxed conditions, I'd have to say from reenacting that 3 rounds a minute are easy. But you gotta remember that the chest on the limber gets lighter after every round. And a 24 pound field howitzer only had 23 rounds in the chest.