I once read in a gun rag (I forget which one) that in general, 10,000rds for each of firearms is good to keep on hand. Since I have several guns, and no million-dollar trust account, I don't have anywhere near that. Right now, I'm going for between 1,000 and 1,200 per. I've been trying to live by the rule, "replace one with two", so for every box I shoot, I buy two to replace it. When possible, I buy by the case to save $$$. So far it's been working, and it seems to be a way to store a little extra up without a really noticable strain on the ol' pocket book. If I continue this, I should be at the 10K per firearm mark in a couple more years (assuming they haven't banned ammo by then). My advice would be to find a load you like, then shop around for the best price, then buy as much as you can afford to. I realize this is off topis, but it kinda fits in; don't forget to buy those extra mags, spare parts, and any accy.s you need as well. An empty and/or broken gun makes a really poor hammer.
-John