Hey Growmie,
Flushing is usually commenced two weeks before harvest. It is best to take a close look at the trichomes on your plant to assess when your plant is likely to be ready for harvest. If the tiny trichomes are just beginning to turn from clear, to a cloudy and milky color, this could be a good indication that the plant can begin flushing. If you flush too early you'll hurt your overall yield. Hopefully, I answered your question and remember Happy Growing and Free your mind one puff at a time.