Okay, you can start flushing your plants when you see that the trichomes are mostly cloudy but just a few still clear- this should put you within 2 weeks of harvest. You don't really need to 'flush' per se right away; simply switch to plain water for that time, and if you want to really do a good flush, do it right at the end about 4 days to a week before you chop.
The goal is to see some significant yellowing of the larger, older fan leaves immediately prior to harvest.