Mummy cat gone missing, how long can babies survive with no milk?

My cat has 3 three week old kittens and she was unintentionally let out of the house by my friends little boy when he was leaving. the kittens were obviously fed before she went out as they babies are still sleeping and quite content but i was just wondering how long they can go without her feeding. i have got supplies for this kind of incident, i have some little eye drop pip-pet things and some kitten replacement milk.. when should i intervene? thanks x