"The FORS2 calibration plan previously had a validity for imaging sky flatfields of four days. This placed some stress on the observers, especially during periods of bad weather. We therefore decided to analyse a series of flats to check if it was possible to extend this validity range, and if yes, to what level. The analysis allowed us to change the FORS2 calibration plan, increasing the validity of sky flats from four to 14 days."