I don't grow in hydro but I think the "when" on nutes for them is pretty standard... I actually start my girls off with bloom nutes along with their grow nutes but in general, you want to start bloom nutes as they go into pre-flower. In pre-flower, I start cutting back my grow nutes and pumping up my bloom nutes (my grow nutes are heavy on nitrogen which is a detriment to the flowering stage).
Good luck!