it really all depends on what strain your growing as indica's will turn faster than a sativa will for you.. just keep an eye on them a few times a week, and try to look at different buds on different areas of the plant too as some areas or buds will finish faster than others.. and its ok to harvest part of the plant that is the color you want and let some go longer..
i would say you could start to flush when most all of the trics are atleast cloudy, and your seeing some amber ones as well.. if you flush for a week or so, by the time your done, they should be about ready..