They should be checked/changed every 1000 miles (per maintenance schedule) to keep your scooter running tops. Over time, roller weights will develop flat spots which will impair the performance of your scooter.
A good indication that your rollers are worn is increasingly high revs for a given speed and/or decreased top speed. Replacing worn rollers with this set will restore and actually improve your low-end power and especially your top speed. Fuel efficiency will also improve dramatically!