DWC ?'s

i have a couple questions about dwc hydro systems.

1. if i were to use a dwc system, how often would i have to switch out the nutrient solution?
2. is the reason you have to swap out nutrient solution because it stagnates?
3. could i circulate the water with a pump to keep it fresh longer?

1: As often as need be, the rule of thumb is once you have topped it off (water only) to replace more than half of the total water absorbed you should completely change the water and re-nut up.

2: I am sure that plays a part in it, but it is also do to how much is actually used.

3: that is a Rdwc as opposed to a dwc. It doesn't keep anything fresher longer, your top offs and nut changes will remain the same, it does however insure the nutrients are well mixed, as in more evenly at all times.

so if i use a twelve gallon rez how often am i going to be changing it because i was told that once the roots are long enough, i should et the nutrient solution get down
to half of the rez and then fill it back up with water. then, when it gets back down to half again i should completely change it out. is this something normal or was i
misled?

and how long does it take for a plant to use up the nutrient solution? i am planning on having six plants feediing from a 12 gal rez.

Small DWCs, I changed every couple weeks. On a bigger, 12 bucket system (80 gallon volume) i used to run, i was very successful witha partial (30%) water change at about 4 week into flower, and the final flush...that's it. With RO topoff water, it's doable...at least it was for me. I wasnt necessarily trying to save nutes or money, I just alwayslet healthy looking plants amd strong growth determine what I did. I treat it like SPS coral reefkeeping...I let the plants and animals tell me.