You should only flush two times. Once between veg and flowering when you switch your nutes up, and the final 7-14 days. Now this is for soil and most soilless. For coco and hydro I have no clue.
Now people can have different definitions of what a flush is. Some people, I think they are crazy, think flushing is pouring 4-5x of water through their soil. So they think a 4 gallon pot needs 20 gallons poured through it, or even just 10.
In my case flushing is simply doing "water only" feeds at a little higher amount then usual. So in my 5 gallon pots at 12/12 switch I usually water only for about a week at about 1 1/2 gallon per plant(I usually water/feed a gallon a plant)
Then near harvest, you need to estimate when this will be first. That itself is something one needs to learn. But at about 10 days before I plan to cut I begin "water only" feed at about 2 gallon per plant.
So basically flushing to me means watering only at a slightly higher amount then usual.