In Carl v. Decker's 1828 compilation of data on artillery: "Der Preußische Taschen-Artillerist" (the Prussian pocket artilleryman), he states (p. 158) that generally soldiers would fire

6pounder gun: 2 rounds in 1 minute
12pounder gun: 3 rounds in 2 minutes
7pounder and 10pounder howitzers: 3 rounds in 2 minutes

If exact aiming is important, in average this goes down to 1 round in 1 minute for guns, and 2 rounds in 3 minutes for howitzers.

With canister (short distances, so exact aiming isn't so important), you could fire with the 6pounder gun 4 rounds in 1 minute, and with the other types 3 rounds in 1 minute. This is valid if the barrel is wiped out only after every third round. (In the Prussian artillery, wiping out the barrel with water wasn't practiced, it was only brushed out.)

This data was collected during several manoeuvers, but reflects what was possible in the field. Decker points out that factors such as the experience of the soldiers and other influences which delay or speed up the firing can't be really calculated.