Generally you’d flush for 2 weeks but it is best to take a close look at the trichomes on your plant to assess when itd is likely to be ready for harvest. If the tiny trichomes are just beginning to turn from clear, to a cloudy and milky colour, this could be a good indication that the plant can begin flushing.
You can get a flushing agent to maybe reduce it to a week but for the cheap option is just to flush 2 weeks with plain ph water.
If it’s an organic grow then no flush needed as plants will do it themselves.
Hope this helps man.