I flushed my girls for 10 days and then another 3 days in the dark with no water (Stress Ripening). During that time they showed no sign at all of any nute deficency. Maybe it is because my lights are a bit weaker than yours, but I would aim for 10 days of flush if you can. I havn't seen any log on here that has flushed less than 10 days and I am going to do move up to 14 days flush for my current grow.