i clean mine out every 2nd or 3rd day,daily spot cleans are also how i keep mine clean,some of my piggies are also litter trained,the best way i have found is to put hay in the litter tray and each time you see stray poo's in the cage place them in the tray as this encourages them to go in that area the same applys with many dogs,a good 50% of piggies wont toilet train but sometimes you get lucky and it greatly reduces mess,
i have also heard people place the hay rack above the litter to encourage them to sit in the litter while they eat therefore making them go to the toilet.the downside is they love to eat the hay out of the litter but its not to much of an issue apart from replacing it.
Sawdust.woodchips,shavings,straw are considered by many to be unsafe due to eye injurys from straw,respiratory problems with sawdust and the roughness of chips on a piggies feet which can reslut in bumblefoot and unless you get ultra soft shavings they can be rough too,the chips and shavings can also be treated with unsafe piggy products so i would avoid all of these as much as possible
i use fleece,towels and newspaper in my cages and the only pigs i have a problem with are Zanzabar and Baker who still refuse to use a litter but insist on peeing all in one corner so it gets soggy daily so i place extra toweling in that corner and it seems to absorb it better.
The best bedding from what i have heard is a mix of newspaper,megazorb and hay x