It depends on the nutes you use. Anything synthetic should be flushed for 5-8 days. Im sure there are people that will debate this so do an experiment for yourself. Take ISKs advice and dont flush one or two or half or whatever, then flush the rest (straight water or flushing agent or R/O water (which works best in my experience) AC or dehumidifier water is usually around 0 ppms also and works great for flush). See what you like better.