Pretty subjective, but you'd want to look at exp/hour in w/e camp/zone you are doing. If a mob takes you a minute to kill and gives you 20%aa vs one that takes 20 secs to kill and gives you 10%aa....
With a group of 6 100s the best I can do is about 20-25 secs on a yellow con. Then there is time to loot the corpse, go out and pull, rebuff time, so roughly about 1 mob every 45-60 secs, if you factor in all that time that is wasted not in combat, even with macros running and chain pulling.
You could cut down on some time with an enchanter that chain casts pb ae mezz...., since most of the non-combat time is wasted on pulling.