We know that Superman does eat "human" food. However, is it really necessary for him to eat, i.e. can he starve to death? If yes, would he take the same time to die as an average human?
Also the same question goes for water.
As Sachin Shekhar stated it all depends on the writer in question, but generally Superman enjoys food and drink but he doesn't have to eat in order to survive.
So, he's kind of like a plant, he absorbs solar energy to sustain his body.
You can read about his solar power ability here
From the DC Wiki
Self-Sufficiency: Doesn't need to eat and sleep so long as he's under the effects of yellow sun radiation. His body stores the energy necessary to negate eating and sleeping for an indefinite period.
His weaknesses are Psionics, Magic, Kryptonite, other Kryptonians, Doomsday (and other villains), a Red Sun, a lack of radiation from a yellow sun, and he can't see through lead.