Assuming you are referring to a newborn rat, they can survive for about 3-4 weeks without their mother. After that, they will need to be weaned onto solid food and will require assistance from their mother or another adult rat. If a mother rat dies, it is possible for another adult rat to step in and raise the babies, but it is not guaranteed. It is best to get the babies to a professional breeder or rescue as soon as possible to ensure their survival.