Yes, it can take 72 hours--or longer. We usually don't recommend that people re-test for CYA, or add more, until at least 4-5 days when adding CYA. If you have a sand filter, you also need to not backwash it until the CYA has fully dissolved. If you backwash, you'll just wash out the undissolved CYA on the filter and have to start over again.