At which stage of flowering should i change to a b...

Hello! Well Nitrogen will be needed during the whole life cycle but the amount isn't the same in veg / flowering and late flowering do the only moment you don't need nitrogen anymore is when you will flush your plants just before harvest. So in veg and early flowering ( the 2 first weeks) they require more an higher amount of nitrogen than in middle and late flowering and that is why the bloom nutrients also contain nitrogen. So to answer your question you will need to decrease nitrogen at the end of week 2 and slowly increase the P and K. I hope it will help you 😁✌️🏼