Crushed coral is a great tool for raising the pH of your water. We use it at 1 pound of crushed coral per 10 gallons of water when mixed in with our gravel. Crushed coral dissolves continuously over time. The lower your pH is, the faster it dissolves. As the pH rises, it slows down how fast the coral dissolves. Crushed coral adds a bit of hardness to the water and it significantly boosts KH which buffers pH from swinging.
We find that crushed coral inside the aquarium may need to be replaced every 1-2 years depending on how low the pH of the tap water is.
If using crushed coral in a filter, it will dissolve faster. This means you'll usually use smaller amounts, but replace it more frequently. Replacing crushed coral after 1-3 months is most common.