I can tell you how I normally do it and you can decide if that is what you want to do. I generally wait until I begin seeing amber on the upper leaves. One or two specks and I start flushing. I only flush for a week, sometimes less. When I grow organics, I don't flush at all. There is always a big debate, flush, don't flush, even a year or so ago one of our founding admins here decided to call it "cleansing". I have never seen anyone flush or cleanse an outdoor grow and can tell you, if you have ever smoked outdoor flowers, the quality is much better than indoor.
That being said, new growers that may have had some salt build up from nutrients and they could probably use a good flush.