What medium are you in? I'm in coco and I'll go all the way through flower. The amount peaks through the second half of veg then steadily drops off week by week throughout flower such that by the end of flower it's a very small amount: 15-25% of what it was getting in veg.
You can use cal mag anytime but it loses its effectiveness the later in flower you get. The stuff it provides to the plants is far more active in early weeks than say mamoth which you start slow in flower and go heavier towards the end. Plants are developing and doing different things during early and late weeks.