Being a disabled vet I have some spare time on my hands. I first tried the Army aerial gunnery manuals and came up with nothing. I then looked into gunnery tables and qualifications and came up with nothing. I then went wider and tried every different FM I could find and came up with nothing. I do know some Apache pilots and gunners but I can't stand talking to them so that was out of the question. I finally went to YouTube. A search of "AH-64 gun camera footage" gets a lot of results. On a lot of these videos the HUD is visible. If the Apache is engaging targets with the 30mm, the HUD will display weapon status and rounds remaining. I watch so many gun camera videos that I now see everything in FLIR mode. HaHa. 9 times out of 10 the rounds fired was right around 20 rounds at dismounted infantry. The sound is distorted but the average burst seemed to be right around 2 seconds. The engagements were just a series of 2 round burst on the target until it was no longer a target. Any target that was more solid than a human was normally engaged with FFARs or Hellfires so I could not get a round count for the 30mm on those. I'll paste the link to the best YouTube video I could find. I'm sure everyone is smart enough to see the round count. I don't know if this helps but I thought I'd try to help. I did not look at Russian gun camera footage to tell you about them.
http://www.youtube.com/watch?v=uvrJzkzaG7E
Below is the name of the video on YouTube if the link does not work:
Afghanistan - Absolutely typical apache attack with Hellfire and 30mm Gun