It's not time-related. It's strictly dependent on your log size.
The basic log size is 500 entries. If your site received (and the tracker logged) more than 500 hits in the time interval you are interested in all you will see is the 500 hits and whatever time interval that covers. This can be a month or a week or a day or an hour, it all depends how busy your site is.
The same applies to an upgraded account, though with a larger log size you will cover more time.