I always added it every feeding in flower with good results, but lately i havent and have seen no issues, jmo. I did start adding pro silicate when i stopped the cal mag but have no idea if there is any relation. Calmag 2-0-0 and PS 0-0-2
It depends on the medium and feeding brand that you use. I grow in coco which tends to lock up magnesium, so I have to add 4ml per gallon of water all the way from the beginning until I get to about 3 weeks out from harvest. Iff you are in certain soils or use dolomite lime then you shouldn't need calmag additive. If you are in any kind of hydro, I would recommend using a small amount, 1-2ml per gallon of water throughout unless you see problems. The reason is that most of the liquid nutrients have very little calcium or magnesium in them because they tend to bind with other elements.