Fresh,
I use to have mostly LPS corals in my tank and Ca depletion didn't seem to be a problem until those corals became very large(over ten crowns per colony), then I started adding Kalk in the second week of the month to keep up with the loss. About a year ago I started to add SPS and now I would say that these corals make up 35% of my tank. Once I added a few SPS then things really started to change, now I'm lucky if I can keep my Ca at 380-390ppm.
Like Brad said above, the only true way to know is by testing. When I had just LPS I would test every second week of the month, now I test every single week
