TBH, there is no need to give it to them at all. I'm glad mine only hav water, especially with 2 of them having CRF - water is far more hydrating. Even with cat milk, Clapton gets diarrhoea and Alice throws up. Now and again I give them a little dob of cream but that's it. I think if cats are used to having milk, they are likely to refuse water so not a good idea in the long run.