20 hours to drop is good.

Now, are you running your filter 24/7, vacuuming to waste everyday, and brushing the pool everyday? You should be.

If the CYA of 100 is right, then the shock level of 25 is right. If the CYA level is 40, you may well bleach your liner. This is why it's best to test yourself and rely on your tests. Ben's kit, the Taylor 2006 or the Leslies FAS-DPD Chlorine Service Test kit are the best ways to be sure. They are all basically equivalent, but I won't go into why I think Ben's kit is the best--all 3 are better than everything else out there. But some WalMarts have a 5-way drop test kit that uses OTO to measure TC up to 5, measures pH, T/A, CYA and hardess, all for less than 15 smackeroos--a bargain.