I switch around toys when they get chewed up or I see they aren't paying much attention to them anymore. So, I guess a few times a year for changing toys (some toys just need to be flipped to offer a new chew surface), about once a month for rearranging. Cleaning toys is done on an as need basis, if toss toys are peed on they might get a vinegar and water bath, or tossed depending on what it is and how chewed up it is. It's really personal preference though you can change up the toys as often as you feel like there is no set timeline.
I also rearrange the cage (move shelves, ledges and stuff) around a couple times a year, makes the cage new and interesting again. Do cage rearranging slowly though so you don't have miss jumps, until you know how your chin reacts to new layouts. Some chins try flying around the cage like they always do, others are more cautious and take their time checking things out.