when to switch nutes?

I am i little over a week into flowering, 6 feminized skunks between 18 ' 26 ' inches. when do I switch to bloosom booster? I am on peters regular right now and all seems good. I was thinking of going with jacks classic BB when the time comes. When and how much and how often do I use the BB? and do I still use the reg. after I start? Thanks