This becomes an estimation game. You have to look at the bud and trichs, and try to predict how much longer she has to go. The harvest week breeders give are not always accurate. I usually try to guess when the plants have about 10 days to go and start flushing.
First, decide where you want your trichs to be when you harvest and go from there. If you want them 50% amber, then you try to guess when you have 10-14 days til harvest.
most indicas dont turn amber they just go brown and degrade!
for indicas the trics are cloudy when ready but they are not an acurate way of telling when a plant is ready as my trics start nto turn cloudy around week4.5-5 i used to use a microscope all the time but the only way to tell when a indica is ready is after about 1/2-2/3 of the pistols have turned brown and the calyxes are fat and swollen.
But sativas do turn amber you can use a scope on them to tell when to harvest.
Some strains even produce purple trics.
I would flush about the 7th week and continue until about week 9 even if it is a 8 week strain....i have learned that if u do let some strains go and extra week especially sativas the results are much better and the buds expand even more...i usaully flush with just plain water with a ph of 7...dont wanna flush too much at one time because the plant still has to live off of the nutrients left in the soil.