how often should the bedding be changed

You should do a full-cage clean at least once a week, where you empty out all of the shavings wipe it with 50/50 vinegar/water solution, and put shavings back in. It should be spot cleaned (scoop out the dirty areas and replace) every single day. You can read this https://www.guineapigcages.com/cleaning.htm for help and other ways to clean, but the vinegar wipe solution is easy and quick. You should also visit this https://www.guineapigcages.com/bedding.htm for info on good bedding.

It really depends on how many guinea pigs you have and how big the cage is.

If you have one guinea pig in a tiny pet store cage, you need to change it daily.