For the last 25+ years, I have been giving/continuing with grow nutrients until there are small budlets formed, before switching to half grow and half bloom for the next 2-3 weeks, before changing to just pure bloom nutrients through until maturity. This seems to produce the biggest blooms on plants that do not exhaust themselves and end up canabalizing themselves to a sickly yellow colour.
I will also give one dose of half grow and half bloom about half way through the "just bloom nutrient" phase as a plant booster, but probably one third grow nutes and two thirds bloom nutes would be a more accurate description.
Cannabis does still need a good supply of nitrogen in early flower and switching to bloom nutrients when the first pistils appear will/can lead to a reduced yield and certainly reduced health in late flower, and seems to be a common rookie mistake amongst a lot of growers on this forum.
The routine I use is the evolution of my experiences growing outdoors for 35+ years and the result of much research and experimentation.
Others may disagree, but it works outstandingly well for me and in my situation.
Hope this helps, Organoman.