If they get the chance, as much as they can! It is important to ensure that they are not fed too much as this places strain on their skeleton and heart and can lead to problems in later life. Small, frequent meals are preferable; four times a day for puppies and twice a day for adults is a good guide. A useful test to check for overfeeding is to feel their ribs. Ribs should only have a thin covering of flesh/fat.
If the ribs are covered by a substantial amount of flesh/fat (more than a centimeter or so) then you are probably feeding too much. If feeding commercial, dry, food the guidelines on giant breed (puppy and adult) should be adhered to. Breeders tend to have strong views on the appropriate diet for puppies and adults and it is reasonable to expect detailed advice upon buying a puppy.
If you intend to feed raw food (BARF-Bones and Raw Food) the following links are useful: