They eat on and off throughout the day. I feed mine a bowl of veggies in the morning, change their bowl of food pellets in the evening, and they have hay all the time.
Guinea pigs are grazers which means they graze on food all day long. You must always have a supply of Timothy hay and water available for them. They will need a bowl of nuggets a day which is a dry guinea pig food. Twice a day you can feed them fresh vegetables.
You're supposed to keep them with hay constantly, pellets pretty often or constantly, and veggies once or twice a day in small amounts. They need the hay to keep their systems running, but watch everything else.
I swear to you mine never stops eating. I think I spend more money of food for her then for myself.:)
When I had them, All day.
wen i feed mine i keep feeding them till they wont eat