what do you grow them in, that's the main question.. if you grow in soil, which keeps a lot of the nutrients stored, then it's best to start flushing about 2 weeks prior to harvest. if you grow in soilless medium, like peat or coco, then a week should be sufficient.. in case of hydroponics with the roots hanging into the nutrient solution (doesn't seem to be the case, looking at your pots) 2 to 3 days is sufficient, because here you're swapping the whole medium completely.. remember you're not flushing the plant, but the medium, so the plant won't uptake any more nutrients and use what it still has left inside itself..
As in "how", there are different methods for the various mediums. As mentioned hydro is the easiest by just changing the reservoir (possibly again with a flushing additive) and it's pretty much done. For soil, coco, peat and similar you can either use regular water (some say they use 5 or 10 times the amount of water that fits inside the container and then rinse it with that.. the best method is to actually measure the EC of the run-off), others just lower the EC value of their nutrient solution at first (to not fully starve them, when they want the most food for bud-creation), and then use something like "florakleen" or "flawless finish" or similar, which is meant to break down the salt-residue that the nutrients have formed.
When you want to harvest depends on what type of high you want.. if the trichomes are mostly milky, then it'll be more of a head-high, while if the trichomes tend to go more amber already, then it's more of a body-high.. so this depends on you.