Unfortunately, there's no schedule or standard increment. Minimums can change at any time and depend on many different factors. In the last few years, changes to the minimums have been occurring in the spring, but apparently that's just a coincidence.
Although they can also change at any time, prices for the various elements of the wedding often change in October, when Disney's fiscal year starts.
If it makes you feel any better, the minimums actually went down when the economy tanked, so it isn't always an increase. Although minimums have gone up since last year, pricing for 2012 is still not as high as it was in 2009, when there was a $25,000 minimum for Saturday Wishes events at certain times of year (they've since abolished seasonal pricing).
