When you create a new monitor, the Preview is a sample of the past 30-days of Twitter data based on your query. This helps you refine your query to make sure that the monitor will pull relevant data.
When you save your monitor, data begins to pull from that moment forward.
If you want to get back data, a historical job needs to be run. We can get data going back to 2006. timeframes for historical jobs can vary from a day to 1 year. Learn more about Historical Jobs here.