I was wondering if it is alright to store prepared Perfect Protein (mixed with milk) for a whole day because I usually make 2 servings in the morning and I drink the last one at night?

We do recommend that you don't leave it for any longer than 12 hours as the flavour and texture can become quite distasteful. As long as you store it in the fridge, then it will be fine. Treat it like you would any milk product. I have been doing the same as you for years and I find the one at the end of the day better than the morning one.