Many folks say 10-14 days. I ran mine 20 days with no problems, as I was adding a ton of water for 4-6 plants like you.

What is the temp and humidity of the growing area? What is the water temp?

You need to do a nearly full flush as some things will not get picked up by the plants and you can get toxic levels..

Are you adding 100% nutrient solution every day? Or adding a lower potency amount (correct)?

I live in Brisbane Australia where we have tropical weather, I have a 200 litre tank of nutrients with 5 large trays, growing Chinese Bok Choy and other vegetables with one tray with a Heirloom Brandy Wine tomato.using the ebb and flow system,The nutrient uptake is considerable, but looking at the size of the tomatos which are still green are nearly the size of my open hand. So I say its okay as long as you get the results.Just think in the garden or in a large pot with daily watering it amounts to about the same amount of moisture uptake.

I top up my nutrients every week with fresh water, and only add nutrients if my EC meter tells me it is required, I also check the PH to ensure it is okay. Having said all that , my tomato plant seems to keep growing like Jack and the Beanstalk fairytale, even though I am growing a mixed crop, it doesn,t seem to mind if the nutrients go up and down, only the nutrient level is hungrily used up.