When the price has dropped. Seriously, today's games drop almost half the price withing 2 months. No, not ALL, but a lot of them. Games like CoD, GTA and Nintendo titles stay strong because of their ability to sell well.
Also, thanks to growing number of stores that are selling games, prices on them varies even more.
Buying at Gamestop is a very bad ide. Simply because of their buy/sell used games. The price on new games in store are ridicules high.