Troy, if you are talking a non-living target then what ever distance you set is ok. If you want to see if you can hit a soda can at 100 yards then go for it. However, if you are meaning live game then the closer the better. No more than 25 to 30 yards max. This would be using one of the more powerful AGs.
Personally, I limit shots to no more than 20 yards. I want good clean deadly take downs. The OSOK. And I do not believe in wounding animals just to have them go off a die somewhere.
And I am lazy and I don't want to have to look for them to finish them off.
I pride myself in closing the distance on game , even squirrels.
This is actually a Good question as I have read comments on the InterNet where persons make some incredible claims as to how far away they shot a bird or squirrel. ( 100 yds etc.) I don't believe it but others may. Plus this sets bad examples for Air Gunners in the eyes of the public. Always be aware of what is behind your target, is it a safe shot? If not, don't take it.