He can go one week without food but feed him everyday or you'll have little rat funeral.
How Long Can A Rat Go Without Eating?
A rat can SURVIVE several days without food. However, rats should be fed daily, including a main diet of fresh lab blocks, supplementary pellet/seed mixture, and a constant supply of fresh water. Also, rats enjoy fruits and veggies, timothy/alfalfa hay, and the occasional rat-friendly snack.
It can die in 2 days but don't feed it often because they nasty and ugly creatures they don't belong on the planet