I have seen in films and series (like master and Commander / Horatio Hornblower) that ships sometimes did practice broadsides on quiet days. To train for more rapid delivery and reloading of broadsides. But wouldn't this be a giant waste of cannon balls, heavy objects that certainly were quite valuable and not a thing to waste? And if they didn't use cannon balls, wouldn't it still be a giant waste of gunpowder?
Question; did they really practice broadsides? With or without cannon balls? Or is this a very rare event that commanders only did in true times of preparing for a climactic battle?
I wish I could remember where we talked about this before, but we have. The short answer is - sometimes. Problem is, powder cost money for good practice, and the Royal Navy only allows for a small amount for practice. Otherwise, it has to come out of the pocket of the captain. But some were willing to take that cost for fast gun crews. And sometimes, they used live rounds with ball and all to do it. It varied greatly from captain to captain, though more of them stuck to the rare live practice (if any) and the dry runs.
As to how fast - if one was just trying to do the fastest rate without considering long term battle, there is the legendary 3 shots in five minutes. But that is not sustainable over a battle that could last hours. 3 minutes to the shot was real good, 4-5 average. Anything faster than 3 would tire crews hauling in the heavy guns quickly enough.
Found the post where I originally talked about this, plus more: How did sailors in the Age of Sail get their combat training?