Well my personal preference is still Flora Kleen from GH but most of them are the save formula for the most part.
With the GH product I flush for 3-5 day's at the end changing the water everyday; but the most important thing to do it to watch the PPM.
On day one for example:
ppm starts with 150 before you turn on the pumps (just the ph adjusted water and flora kleen) Check the ph a few times a day it will change on you often sometime.
Day 2 before you change the water out check and record the PPM it will be somewhere around 500 or so.
Change the water and flora kleen again (starting at 150 PPM)
Do the same thing as before check the Ph often and keep in check.
Day 3 check and record again the PPM before you change out.
This time it is probably around 350 PPM.
Now you change and check every day until you have less then 50 PPM change over 24 hours.
It can take anywhere from 3 to 5 or 6 day's depending on the nutrients you used or how hard you pushed the K at the end too but it works.
That is a proper flush and the way I personally track and flush anything I grow in Hydroponics.
I hope that helps.
If you go cheap you grow cheap. - Me