How much and how often depends on you particular tank consumption which will increase as the coral population increases and grows.
If levels fall too low coral growth will stop and in extreme cases the coral tips will deteriorate as not only they can't deposit the calcium carbonate they need but at certain poit it will dissolve.
Usually before that point is reached your PH will be so out of wack that the PH issue will have bigger and faster repercussions.
The recommended levels for Calcium will be 380 to 450 ppm, alkalinity 3 to 4 meq/lt and Magnesium 1250 to 1350 ppm.
Until you get more familiar with it and know how your tank reacts I will recommend to start with a commercial two part supplement so in case you have adjustment problems you do not have to guess if your DIY supplement is at fault.(B-Ionic, Two Little Fishies or Warner Marine will be my preferred).
The two parts will have to be added in the same amount as they are designed to add Calcium exactly in the same proportion as the required Alkalinity. The addition of both is required except with those times were you need to adjust one parameter and not the other. If say your Calcium is low and your alkalinity is high, you can add only the Calcium part and vice versa.
The amount of two parts to add has to be determined by trial and error, in other words, test your level of alkalinity, add the manufacturer recommended amount and test 48 hours later, if dropping increase the amount and viceversa until you find the amount that will keep the alkalinity stable.