As long as they are with their mother, kittens, like all young mammals, will feed at intervals throughout the day and night. In the first week or so they will do almost nothing but feed and sleep. If you are raising kittens without a mother, you will have to give them some night feeds at this stage.
When they are about three weeks old, they can be introduced to milk or water in a saucer, and encouraged to lap. At about this time they can also start eating solids, or at any rate special kitten food (available in tins from most supermarkets) usually mixed with a little “kitten milk” (adult cats shouldn’t have much cow’s milk, and for tiny kittens it’s best avoided altogether.)
Once kittens have got the knack of feeding themselves, they often have enormous appetites. This isn’t surprising if you remember that they double their birth weight in the first couple of weeks of life, and grow extremely fast for the first few months. Normally their appetites will slow down when they are a little older – if not, do beware of overfeeding.