...yeah man, you can't find an answer because there is no single answer, or maybe the correct answer would be 'you add it when you see your plant's need it', there are just too many variables that effect the answer that there is no way to answer.

i suggest you post up some pictures of your setup and describe, in some detail, your chosen style of grow, what media you are using, what your feeding regimen is, etc, etc, etc, this way experienced growers will have a better chance of giving you advice that is actually applicable in your specific situation.

peace, SOG

edit: i see now that you are in DWC so determining when and how much cal/mag to add is still subject to your water quality, if you have hard water you need less cal/mag and that less often, if not you need more and more often, ...this is why experience is your best teacher, learn to read your plants and they will tell you when and what they need.