21 days when using nutes in soil...I hate to harvest green looking plants. I hate using products to flush plants, it does't make sens to me. Water and strickly water! Best water to FLUSH imo rain water or RO, better than tap anyway...
(But when growing organic tap water is better for regular watering of course, it contains Cal Mag and much more which hard to get into organic watering...)
In fact the number of flushing days mean nothing, its more how many times did u water with plenty water only. And if ur pot are pissing that water or not...
Supose i flush for 3 weeks, but in fact water only 4 times, it won't be as flushed as someone who will water 7 times in 15 days.
And that depends on how big ur pot are, how fast ur soil dry, how much water u add there, how big is the plant etc...
The best herb i have smoked in my life were the ones grown organicly with 70 to 100% water during the whole grow. This is the herb that dries, cure and smoke the best IMO.