Nutrient mixing for soil

I do know that hydroponic growers have to replace the solution in their reservoirs periodically. Since I hand feed with soil and premix my nutrient solution by the individual gallon, how often should i dispose of old nutes that are mixed and sitting in a large jug. Or should the mixture be fine to sit around?

I'm using the AN three part system. I know that you have to add the "bloom" solution first before adding the rest. This lead me to wonder if by letting the nutes sit around, would the chemical properties change with time.

Always add Micro first then wait a few minutes.

Sweet man.... wonder why though