In graves and sarcophagi, corpses all decay at the same rate no matter what the exterior conditions are. This means that corpses will start to rot after a short amount of time, then will slowly rot away into skeletons (takes over a year), then eventually disappear. If you want to keep corpses (people or animals) fresh, which as stated in @n_palum s answer, is the only way to get meat off them is to keep them fresh, which turns to rotting after roughly a few days in game, or to refrigerate them in sub zero temperatures (sub 32 in Fahrenheit I think).
It is worth noting that unless your colonists have the cannibal trait, they get a serious mood de-buff (don't know how much atm, cant check, will edit) when eating anything that is or contains human meat. If you do have cannibals, store corpses in a freezer and make sure other colonists don't see or eat the corpse. Apparel (clothes) that has been worn for any amount of time by a corpse also gets the dead mans apparel tag, which also applies serious mood de-buffs, so I don't recommend using graves as storage for clothes either.