I know the ph doesn't change cause I'm doing that right now but I was also curious as to how long you can let mixed up nutes sit. I mix a gallon at a time and it lasts about a month depending on age and strain of plant. I keep them in sealed bottles sterilized with bleach water and rinsed prior to filling.
As long as you shake them up every time you use them to get the nutes that have settled on the bottom back into solution, you'll be fine. If you were to let stuff sit around forever, the only problem I could see would possibly algae.