How often do you change water in your hydroponic DWC systems?

Hello, I'm going to start growing in hydro DWC. It will not be a loop system but individual containers with air pump/stone. My question is, for 1 plant per box container and 5 plants per box container how often should I change the water (weekly, biweekly) and how much each time? (%)

Also, while we're at it, for a setup like that, how much ferts from General Hydroponics are you using on each box container? For example, the instructions for General Hydroponics tell you to add, let's say, 4ml of their N solution to 1 gallon. So, if you have a 5gal container do you add 20ml of the N solution? or you just mix 4ml in 1 gallon and then add it to 4 gallons of water already in the container?

I ask this because I'm using those ferts right now on soil and for 1 plant I use half (i.e., 2ml instead of 4) in half a gallon water and that half gallon of fert lasts me 1 week of fertilization plus regular watering over the week.

I want to know your typical maintenance schedules. Please.

A good rule of thumb is change when they have drank 50% of the nutrient solution. I don’t use general nutes but for seedlings in dwc a good place start is at about 200 ppm and raise it to around 500 ppm over the next 2 weeks or so depending on how they react. For seedlings I have the water level just low enough to not saturate whatever you started the seedling in. As the roots grow I let the water level drop to about 6” below the top of the bucket.