Commercially prepared, unmixed nutrient formulas often don't reflect an expiration date, which means that so long as they're properly stored, they're good pretty much indefinitely. Once a nutrient solution has been mixed, however, there is typically a fairly short shelf life - approximately a week, if stored appropriately. The act of mixing will incorporate all sorts of microorganisms that are always present in the air, and readily available nutrients will quickly encourage rapid growth, if their other needs are met. Algae and both beneficial and harmful bacteria can quickly establish themselves.
Not only can those bacterial colonies cause problems (especially the anaerobic variety), but they'll actually consume the nutrients that you've carefully prepared. Microorganisms also have short lifespans and you don't want decaying organic matter (and more microorganisms) flowing through your system.
