I think you have hard water and with the caveat Seattle_Aquarist mentioned, you do not need to add major amounts of Ca , Mg or CO3. I found certain species did better when I increased my GH from 4 to 6 (like Hygrophila, Sagittaria and Myriophyllum) but take them as random observation not as establishing cause - effect.
To answer the question about timing...
a) dose at water change -target a major increase to match the previous/ideal aquarium water. Most people dose at this point if they have very soft water or use Reverse Osmosis (RO) water (KH~0 GH~0) (not your case)
b) with micros : Mg is part of many commercial solutions of micro nutrients. (JBL Fe+trace elements adds 3.3ppm Mg for every 0.13ppm Fe )
c) daily or with macros : as part of EI, the GH Booster was invented for people with very low GH. Plants use some amounts of Mg and Ca, especially if they grow fast and the GH Booster sol. targets a small addition of Mg Ca and K to be dosed regularly. There is no one formal description of ppm for this solution, various nutrient outlets offer their own mix. The description from nilocg website says Magnesium Sulfate, Potassium Sulfate, Calcium Sulfate(1:3:3 ratio)...whatever that means molar, mass or volume ratio ?. GLA Ultimate GH Booster has a nicer mix imho at 3,2ppmK , 1,66ppm Ca and 0,23ppm Mg
d) when /if needed. In waters with moderate to high GH, Ca is abundant enough. Mg might be missing but is easily spotted as white patches in old leaves while veins are still green. That and the fact that I have stopped seeing any deficiencies at 3ppm Mg.
As you can see you have plenty of playing room with these two nutrients. In aquariums for which I use RO water I target a KH of 4, GH 6. I find this is enough to not have to bother with the GH Booster even for a high light tank.
Hope this helped.
On hiatus till later this year