if your water is properly maintained (lightproofed) at the right temps, 65-75 with 60 being lowest and 75 being highest. i think that 70-72 is optimal for chemicals because i think 60-70 is optimal for microbial life, correct me if i'm wrong though. So yeah basically to tell if your water is OK you should already have air stones in the res yes? Well what i do is i pick up the stone and feel it, if it feels not slimy, then i think its still fine, although your goal is to change the water before it gets slimy because you dont wanna give your plants slime-water... to prevent slime also i hear pondzyme works well or theres more products like great white and whatever.. but for each you must learn how to use it correctly. Possibly you could cap your ebb and flow to reduce the amount of evaporation during floods, and light hitting the water, some people use that insulation reflective material and it blocks the light and reflects it back , thus increasing light concentration.
so my answer is try to change the water weekly, and thatll make up for those times where you forget to change it the 2nd week. Is your res big enough to handle 2 weeks? if it isn't you risk the res drying up and your pump burning out from no water.
also some people insulate their resevoirs, so all the heat from the day cycle doesnt heat the resevoir as much.
hope this helped!