Wait until there are 98-99% brown pistils, this is the harvest time and will put you in the "10-30% amber trichome" zone without having to look too closely and is also when the plant has peak THC, which is what everyone wants.
Start you "flush" about 10-14 days before this, when your plant is at the 95% brown pistil stage.
Following these guidelines is by far the easiest recommendation for new growers rather than trying to figure out exactly what is going on with trichome colours, which has many variables in my experience, all linked to genetics - like never getting amber trichomes in some strains and other strains having 70% amber after only 4-5 weeks of flowering...... all because of genetics.
Going by this picture, you probably have 3 weeks or so still to go.
5 weeks of flowering is only about two thirds the way through......
Hope this helps, Organoman.