While cats can hold out without food for a surprisingly long time, they can stay alive without water only a few days. They may need only a little water, owing to their ancestry as desert animals, but make sure your cat always has access to fresh water, especially if you feed her dry cat food. Though canned food already contains the necessary moisture level of 78-80%, she'll still need available water in a separate dish.
Cow's milk isn't a suitable drink for cats, since it contains milk sugar (lactose), which can cause diarrhea. However, there is some special lactose-reduced cat milk commercially available, such as Whiskas Catmilk + Plus.