Kinda depends if you’re running autos or photoperiod plants..
Either way tho, usually shift to the first week of flower nutes once you start to see formation of clusters of stigmas/pistils around the bud sites.
If you’re growing photos, this is initiated with 12/12 lighting cycle and occurs usually 10-14 or so days after flipping.
I usually feed normal veg nutrients for the first week after switching to 12/12 then begin the first flower feed that second week and on.