As much as they need! Guinea pigs need hay to aid their digestion – like us humans they need roughage. They’ll eat a lot of hay, so it needs to be pretty much constantly available to them, either as a snack attached to the bars of their cage or a bedding-come-snack in their bedding area.
Hay is very important for guinea pigs
You can buy special extra-nutritious hay from most good pet shops, which you can put in their hay dispenser and keep separate from their normal hay bedding. This is sometimes enriched with extra vitamins and minerals, so can be a good option if you want to supplement the vegetables and dry food.