Study time depends entirely on the individual.While true, how long does it take to cover those obscure things? You don't need a class for it. At worst it's probably $50 for a book and a week to read what you need. You don't need to ace the cert exam..just pass it.
Those obscure things might take you a few days with a book, but someone with other obligations or poor time management skills might take longer.
Granted, you shouldn't be taking semester long classes, or boot camps just to brush up on material you already know. If you already know most of the material and are just studying to refresh and cover the few topics that are new material or items that you don't do in your daily routine your study time will be far less.