Twitter is currently limiting most search results to tweets from the past 7-8 days. This varies from report to report, but most reports will include tweets that are between 6 and 8 days old. Please note that snapshot reports will include all tweets currently available in Twitter's Search API, which is limited to approximately 1,500 tweets per report. That means high volume searches will go back less than a week - they'll stop when they hit 1,500 tweets.
If you'd like to check on a particular query, just ask us!
To be safe and ensure you get all your tweets, we recommend running reports as soon as you can after your event (please don't wait a week and assume they will still be accessible). When in doubt, run your report within 3 days of your event. And if you want longer-term tweet archiving and analysis, set up a Tracker.
Note: When you run a free report for a term, we'll estimate how many total tweets will be in your full report (up to 1,500, which is Twitter's limit on a single query). That number will be in a blue bar at the top of your free report. If you have any questions about that number or your specific query, just ask! And if you want to access older data - going back to 2006 - request a historical analytics quote.