How often can I give my cats kitten milk?

I bought some cat/kitten milk a while ago, and the cats really seemed to like it! I wouldn't mind giving it to them every day, as another something they can enjoy (seeing as they're inside kitties so I feel bad for them and I want to spoil them to make up for it). But is that okay? How often can I give my cats kitten/cat milk and how much of it is okay to give them, if I'm giving it to them daily? It is milk that's meant specifically for cats, I wouldn't just give them cow's milk of course. Thanks for your time. 

How old are your kitties? There's no harm in giving cats a little bit of cat milk as a treat, but I would not make it a large part of their diet. Also, don't forget to count those calories into their daily intake. I think the cat milk can be fairly high in calories.

One of them is about a year old, and she's the mother of the 4-month-old kitten. I am not worrying much about calories for now, because the kitten has a lot of growing left to do and she insists on still suckling from her mother, so Mama cat needs more energy than she should need right now to suckle baby. They were strays when we took them in, and Mama has just in this last week stopped being skinny and gotten to a size that looks healthy. I'm more concerned about it giving them digestive upset, or something along those lines?