If you are able to mathematically predict the chance that a wager will win, then you'll know exactly when to bet and how much (see past threads about Kelly betting). If you don't, you will need a strength threshold that "tells" you when to bet a game or not. If keeping yourself to one play per week is what you need to feel good about a game, so be it, but there really should be no limit to when you make your plays (what Matt said).