Depends on the high you're aiming for. If you want that couchlock body stoning, let ten percent or so of the trichomes go amber on you. The longer you wait, the more of a couch lock it will be. If you want more of an 'up' high, harvest a few days before that point. Keep in mind the 'couchlockness' also depends on the strain, and Indicas are naturally more of a couch lock than Sativas, so there are limits on how much you can affect this by altering the harvest time. I usually harvest when there are 5 percent amber and most are cloudy, personally.