Like people, they live longer if they don't get too fat. Similarly, each will metabolize what they eat differently. The best guide is how your rabbits looks and feels. My basic guide to tell if one is too fat is to see if I can get a handful of loose skin over its' back. If I can feel every bone down its spine, it's too thin.
Some breeds tend to be beefier than others.
If you buy from a breeder, ask to feel what a good weight rabbit should feel like, then go from there. If you're feeding the scheduled amount and your rabbit feels too thin or too fat, adjust the quantity accordingly. If he still feels too fat or too thin, consult your vet to rule out parasites or a metabolic disorder. A rabbit can live for quite a while without food but it cannot live without water . Lots of fresh water should always be available.