AdWords allows you to look at the performance of a keyword for today, yesterday, seven days, this month, last month, etc. What period of time do you use to decide if a keyword is giving you what you want? I usually look at seven days or sometimes up to 30 days. Anything less than seven days is not enough information, while I think that more than 30 days won't help because advertisers come and go. What works best for you?