How long can I leave a pre mixed Nutrient reservoir

I am trying to set up a nutrient reservoir so if I ever need to go away I can set it to water but how long can I pre mix it before Minerals start to fall out of suspension and my ph changes it self ECT… I plan on using a UV water purification to move the water around and keep the Algae down. when I get the funds I will use a large air stone to Agitate the water as well but for now I will use a water pump