the main question is what medium are you growing in?
if you're in soil, then you should start flushing about 2 weeks before harvest..
when it comes to soilless (coco, peat etc.) then about one week
with hydro that grow in a solution usually 2 to 3 days is sufficient, as you don't have any salt buildup in the medium that needs to be "flushed".. you simply exchange the whole medium by doing a reservoir change..
i guess when is the right time can get a bit tricky if you don't have too much experience with how fast the trichomes change color..
also there is no "definite harvest time", as it also depends on the type of high you want.
The more amber trichomes, the more of a couch lock it will be.. the more cloudy ones, the more of a head-high.. that's of course next to the type of strain (sativa or indica), which also play a general role in the type of high.. so if you grow an indica, but want a head-high, you're better off harvesting when majority of trichomes are milky and not yet amber..
on the other hand if you have a sativa and want a body high, you'd have to wait even longer for a high percentage of trichomes to turn amber..
eventually it depends on your preference and what type of high you want to get..