My advice is to buy them when they're cheap even if you think you've got enough, but what is cheap? No sense stocking up on Sig mag.s at $80 even if there's a momentary drought (just ask anyone who bought $80 PMag®s). Sure, buy a new gun, get a couple extra at regular retail, but then you have to begin the hunt for cheaper. This requires finding out the going rate, and keeping at least a little connected to the market, so when a store has a sale, someone puts up a good price on the EE, you're at a gun show, you're ready to close on a good deal.
As long as the price is good, you never stop buying; extras are usually easy to sell, if you didn't over-pay so you don't feel compelled to price them too high.