I suspect it depends what kind of cheese the whey came from. Whey has all of the water soluble components of the milk. It loses the casein and fat. How much of the albumin and lactose it loses depends on the cheese. If the milk was heated enough (190F I believe) then the albumin will denature and not be in the whey. If the cheese was acidified with a bacterial inoculation then the bacteria will have converted some of the lactose into lactic acid which will both be in the whey. Riboflavin is water soluble and is what gives the whey that greenish tint.
Either way- most of the nutrients that bacteria like are not in the whey and it is always highly acidic. I have personally kept whey for a couple weeks with no ill effect.
I usually use my whey soon by making ricotta. If the cheese it came from didn't denature the albumin in the milk then you can gently heat the whey until the the albumin precipitates out. A gallon of whole milk makes almost a pound of mozzarella and then a little less than a cup of ricotta. That's a lasagna right there.