According to Wikipedia, the Great White Spot "is somewhat periodic at 28.5–year intervals, when Saturn's northern hemisphere tilts most toward the Sun." Wikipedia then gives a list of observations, the years being as follows: 1876, 1903, 1933, 1960, 1990, 2006, 2010. Discounting 2006, which seems to be something of an anomaly, that's an average interval of slightly less than 27 years.
© Haydn Thompson 2020