There is no definitive answer to this question, as the depth of the base will depend on a number of factors, including the type of flagstone being used, the size and weight of the flagstone, the climate and soil conditions in your area, and any other buildings or structures nearby. However, a general rule of thumb is that the base should be at least four inches deep. If you are unsure, it is always best to consult with a professional contractor or landscaper before beginning your project.