With those numbers, once a week. And yes, you'll see nitrates plunge, then the phosphate leaching out of the rock. How much time does it take? I'm sure someone could be able to make an algorithm to guess based on poundage, but it's somewhat of a guessing game depending on how much was in the rock to begin with.
Get some lanthanum chloride or some calcium nitrate to make it easier. Once you get phosphates down, then you'll need to pay attention to the foods you feed and the nitrate vs phosphate amounts. As you progress you'll get the hang of it. Just be sure to ease off the dosage when nitrates go down. If you test and have zero nitrates, reduce dosage by half, then test no3/po4 every other day. It'll make more sense once you get there.