Due to API limits from Twitter, we can only search for tweets that are up to approximately 4-7 days old (measured from the day you run the report). We can't access anything older than that. And that number is only an guideline - depending on Twitter's performance at any given time, we may find more or less than that. Check here for our current tweet time limits.

If you want an estimate of how far back we can search on any given term, first search for that term at search.twitter.com - we'll find approximately the same amounts of tweets for the same period of time as Twitter Search. 

If you're going to be running a long campaign, you should probably run a report at least once a week to make sure you don't miss any tweets. In addition, we are limited to pulling back only 1,500 tweets on a search term, so if you're running a search for a popular term, you should run reports more frequently. There's more info about our search limitations here.