I average about 1,000 rounds of .223 Rem and .40 S&W. When I get down to around 500, I start looking for another 1,000 (or the components to make that many). One thousand is enough for several months of shooting for me.
I don't shoot other calibers as much and have less on hand. But the principle is the same - enough for several months and look to buy that much when I drop to 50% on hand.