I start using it whenever I switch to 12/12, but it isn't crucial to have it happen at any specific point. There isn't usually too big of a difference between bloom formulas than regular growth, but there is usually less nitrogen and higher P and K. You can use reg 20-20-20 throughout the grow from veg to flower, but lowering the nitrogen is usually recommended during flowering. The difference is minimal... I prefer a bloom mix because my fan leaves yellow and drop during flowering (Sativa) and the lack of excessive chlorophyll helps to keep the flavor less harsh, IMO. This is sort of the reason that some flush their plants, but this is really only needed if you are using a ton of food/nutes. Less is best when it comes to nitrogen during flowering.