More or less it depends on cantainer size but I say the last few waterings before harvest is the time to flush and should have next to clear run off. 3x the pot size, 1 gal pot=3 gal water
I'm ending week 7 of flowering this Saturday. I was going to start flushing then and hope my buds aRe ready between week 8-9. Does that sound right to you? I'm growing white widow in rockwool and it says harvest time is 8-10 weeks.