As a basic rule of thumb reservoirs should be flushed, meaning emptied and replenished with fresh solution every 5 to 7 days, no more than 10 at the extreme end. Growers commonly use EC/TDS meters to measure a solutions nutrient levels and top up their nutrient solution when levels get low. The problem with this is that EC/TDS meters only measure overall nutrient / salt levels and can not tell you what specific nutrients are present. Nutrients are used at different levels for a myriad of different and complex variables.
An accumulation of unused nutrients can eventually build up to toxic levels [See: Salt Buildup]. In addition nutrients that the plants may need may have been completely exhausted . Flushing and replenishing your reservoir on a regular schedule is the only way to ensure your plants receive the optimal nutrients without risk of toxicity.
In between Flushes it is advisable to top off your reservoir with fresh water to replace that which has been lost to evaporation and plant uptake. Evaporation only takes up water, it does not take up nutrients into the atmosphere. So as the water evaporates and levels drop the concentration of nutrients does not decline, the nutrient concentration actually gets stronger / more concentrated so do not add more nutrients at this point only fresh water. When adding fresh water adjust pH accordingly. I like to keep a mark on my reservoirs to indicate the appropriate water level. You should also be keeping track of how much water you add.
If you add 10 gallons of water to a 20 gallon reservoir it's time to flush and replenish regardless of whether or not you've waited the 5 -7 days recommended. Evaporation does not necessarily occur at uniform rates nor does plant uptake of water so 5 - 7 days is certainly not written in stone.