Flushing needs to happen all at once. That being said I like to saturate my growing medium with water, then let it sit for 20 minutes or so, then run the rest of the water through it. This allows the water to saturate the plant and soil and make the nutrients in these places mobile and available to be washed away, giving you the best flush.
All this being said, what are you flush for, nute overdose? Harvest? If flushing for harvest, many people here are split on the subject, cause well, there is no right and wrong. Alot of people find simply watering the plant the last two weeks is all you need to do for some smooth tasting bud, no flushing needed. Some people swear by flushing two weeks out, while others like to use nutes untill harvest to get the buds bigger. Personally I like to nute till 4 days out, then give here a thorough flush and thats that, wait for her to soak up that water from the flush then I harvest.
So what I'm saying here, is if you are flushing for harvest, experiment a bit, find out what works for you and what YOU like. That will be the right way to do things...
