You never have to change that water if you dont want to. Many using a reservoir will go 3-4 harvests before actually breakin down and cleaning a reservoir. As long as you have a good cleaning additive and H2O2 life is good for Ebb and Gro'ers!
The big worry here seems to be "Oh, water evaporates, so it'll screw up your nutrient mix". Truth. But the fix for it is simple. ADD WATER and maintain the same ppm. You can manage pH too right? When ppm gets too high, consider water your "ppm down". =)
just keep it in the fridge