You have to check the trichomes with at least a 30x microscope for harvesting/flushing... Plenty of info around here about that. It's very much personal preference, but your harvest window starts when all the the trichs are all cloudy, and thc begins to degrade at about 80/20 cloudy/amber

Soil takes longer to flush than hydro does. I think for hydro you only need to flush with regular water for a couple days... not sure maybe some other hydro growers can chime in.