I like to add back to my reservoirs based on what the plants are using. I like to run as big of a res as possible to reduce fluctuations and how often I have to maintain it. My last set up was every 2-3 days.
If the ppm of my res was stable then I new the plants were eating and drinking pretty evenly. So I would add back the same nutrient mix to will the reservoir.
If the ppm had dropped drastically it would show me that the plants were eating more then they were drinking and I would add in a slightly stronger mix and raise my overall ppms of the res.
If the ppm had risen drastically then the plants were drinking more then they were eating and I would add back plain water to balance out the levels again or sometimes even lower the overall ppms if the plants continue not eating as much.