Whenever there is one worth buying or im not being nostalgic for old games. In the last month Ive bought mafia 2 + dlc and amnesia, before that is was mw2 quite a while ago. Usually only find about 3-5 games a year worth playing or that grab my interest (new games anyway - spend a lot of time hunting down cheap old ones i.e. mechwarrior, giants, ff8 etc). Rest of the time im replaying through old games (have around 200 back catalogued from bioforge to bioshock and from deus to deus ex).