I tell customers that if they sew every day, they need a service at least once a year. But never go more than 4 years without a service.
When we do a service, we remove the outer covers and blow the lint out of the gears. If we don't, the lint can build up and damage the gear set on that Bernina 430.
Also on older machines, the old oil can turn into a tar if not oiled deeply on a regular basis, then your machine could freeze-up.