The amounts given on the bags are intented for a very low veg diet, so you need to adjust them. You also need to take into consideration that guinea pigs don't need the same volume during their whole life. Too many pellets are one the main reason for over weight in guinea pigs, which can be an increased risk in an operation.
I would give unlimited pellets to fast growing youngsters, but rather than just topping up the bowl, rather feed in several portions throughout the day and check the bowl in between. Youngsters can pee and poo in the bowl, and it can cause an unpleasant mess at the bottom.
Once the weekly growth rate is slowing down at a few months old (the time differs quite a bit), you can then start to reduce the amount very gradually until your piggies are fully mature adults from about 12-15 month sold. By that time, they need about half to one handful (10-20g). You up the amount when a piggy has been ill and lost weight or during cold winter times when the overnight room temperature falls and give less when your piggies are eating plenty of rich, fresh grass.
