Flush really means two things:
1) during the course of growing in hydro, flushing the entire system and then re-setting it with new nutes. Some do that several times during a grow, some do it weekly.
2) a final flush before harvest in which the plants are left in plain water. Some do this 2 weeks before anticipated harvest which I think is foolish. I doubt it accomplishes anything at all but if you do it don't do it more than 48-72 hrs. When to start it is a problem, just make your decision based on trichs when you think it is almost ready, then flush.
My still unanswered question for flushers is:
where is all the stuff that is supposedly being flushed out, being flushed to?