2-4k is what I usually dump on skirmish days. but that's a lot of shooting at nothing waiting for people to get organized on the field. remember, if you have low caps and only a handful of mags, 2000 rounds is a lot to reload.
but, when I shoot the M4, I bring 12-15 (field 11+1) midcaps all in the 130-190 round range so it's mighty easy to dump a mag at a tree or something else on the field but still have enough ammo for a few games. Reloading is even easier with the help of an electric speed loader.
although, packing that many p90 mags is going to be a tough task. :P