it all depends on how your flushing. with just water? a week or more. with water and something like florakleen? 5 days. some people will tell you otherwise... there's no precise answer to this question.
I usually wait and flush 2 weeks before harvest then give it straight water for a week and a half solution of floranectar the last week its just a sugar solution to give it a fruity taste and makes your buds sticky
You nee d to wait at least a wk to give the plant time to use up the stored nutrients it has. Thats the whole purpose of flushing to remove builld up in plants.. If you dont wait basically all your doing is washing the roots.