when should i start to flush

hi on my firts crop my critical mass are fin in 3 wk my blue widow 3 wks and my great white shark 4 wks when should i start to flush them and for how long plus i've been to tht after i flush them not to feed them anything for a wk what should i do

plenty of posts on the subject,i personally flush with ro p.h balanced water the last weeks to harvest.... also no nutes

im flushing with budswell a bat guano tea and its adding some funk to my Super Silver Pineapple