As soon as you have something in the tank starting to build calcuim deposits. Most likely the first significant consumer will be corraline algae, but it depends what you have in your tank. Certainly any corals you add will consume Alk, Ca and Mg as long as they are healthy. Even softies consume some of these, just not as much as LPS and SPS. Inverts will also consume this as they build their shells, even Hermit crabs for their legs and claws.
I set my RSR 250 up on 23 Sep 17 with live rock with inverts soon after. First fish went in 21 Oct. Then a couple of early corals. By the end of Oct I could see Alk and Ca starting to drop a little so I started dosing in small volumes. As the tank matured and I added corals the demands drew and I increased the dosage as needed to maintain constant parameters. I now dose 46 ml/day.
My newer RSR 525 was setup on 1 Mar 20 with mostly dry rock, seeded with some live rock from my RSR 250. I started adding fish mid Apr along with a couple of LPS. By the start of May I saw Alk start to significantly drop so I started dosing. Initially I dosed more Alk than Ca and Mg but as things stabilised I moved to my normal 3 part dosing at the same levels in Jun at 30 ml/day. It is now up to 70 ml/day with a few more LPS. I expect corraline algae is still consuming a lot of it in this tank but that should stabilise after 6 months or so when those algae levels start to become more constant.
FYI I dose Aquaforest 3 part.