Aeroponics systems use so little water that nutrient solutions can usually be prepared in small batches and you'll have little need to change it. If you mix enough to keep your system going for a week, then prepare a new batch, you're in a pretty good place. If you must mix up bigger batches, replace any leftover nutrient solution at least every four weeks.
If you're using a low pressure system, where there's a lot more dripping, and recycling the excess, it's important to remember that the water chemistry changes as plants use up nutrients. Be absolutely sure to test your nutrient solution on a weekly basis and make adjustments to reach optimal pH and EC. In most well-calibrated high pressure systems, the solution is so efficiently absorbed that there's no excess to recycle.
