High Doc
I've gone whole grows in DWC without changing the rez even once and got as good or better results as changing a few times.
I use RO water only so I don't get any mineral buildup in my tubs from what's in tap water. If using tap water you should keep track of how much water you add back to keep topped up then when you have added back as much as your system holds you drain and reload with a fresh batch of nutes. Using tap water raises your ppm each time you top up so it makes it harder to tell how much your plants are actually eating every day.
There is no need to change nutes when your plants are small as they eat very little. I just add small amounts of nutes as the ppm goes down to keep it at the level I want or raise it as the plants get bigger and can handle more. Blindly following the makers schedule wastes a lot of good nutes and sends you back to the store more often to buy more. Would almost think that is what they want you to do.
Here's a good article to read about nute management.
Nutrient Sol'n Management and Longevity
Good luck!