StoneColdNTO, i clearly understand your point.
Only, something that I'm pretty sure of is that if you leave your drink overnight in the fridge, I will be good tomorrow.
As the question about the stability over a longer period of time, then it can be different.
You dried protein powed will have a great advantage of aqueous protein solutions. The fact that it is dried will prevent any bacterial growth.
But this can be easely solved for liquid protein solutions if you take care of the sterility. However, it is easy to see that this would be hard to do for you at home, make sure everything is bacteria-free.
If you want to look over stability over a long period of time, proteins should react the same way as drugs (many of them are proteins). Different factors can affect the stability of your protein solution, such as temperature, light, humidity, oxygen or carbone dioxyde presence. Still, this is not a big problem for liquid protein solutions. You make an opaque sealed container and you try to keep it cool. (I don't know much about liquid protein solutions handling, does the manufacturer suggest a temperature at which the solution should be kept ?)
Even though, theses factors can be controlled. But, what I think might happen to these solutions over an extended period of time, is that the amino acids orientation changes.
Your AA can be either L or R, just as L-Glutamine or R-Glutamine. The human body can only use L-AA. Because water is a polar solvant, thus it can allows ions exchange, your AA might over a long period of time changes from L to R, which might diminish the available quantity of protein in your drink.
That's the only thing I see that might happen.
