How long will a protein mix last before it isn't any good? Should I always mix it as soon as I want to drink it or can I pre mix a few bottles then store them in the fridge? I'm talking about using the tubs of powder, not actually buying the pre-mixed bottles like the 6 packs of Muscle Milk.
When refrigerated, I would expect it to last at least 48 hours. If not refrigerated, I would suspect that you wouldn't want to leave it sit at room temperature for more than a few hours.
For lunch, you could try freezing it overnight and taking it out in the morning, then it should stay mostly cold until lunch time. Although, @Salsero69 's suggestion of measuring the powder then mixing when needed is probably the best solution.