Welcome to TFP. I'm not sure I really understand your question. We advocate maintaining your pool water with good accurate self testing and feeding the pool what it needs. Putting a dose of "shock" in the pool is generally something we see no value in since your minimum FC is dependent upon your CYA number.
How fast the chlorine levels drop depends a whole lot on the CYA level, how much sunlight is on the pool, and how high the chlorine level was. If there was any algae in the water will also affect it. A green pool would eat chlorine almost as fast as you can pour it in.
Unless you see or feel algae, or the CC level is too high, there's no reason to "shock". Just raise the FC back to where it needs to be. We don't "shock" around here anyway -- that's pool-store talk for a massive dose of chlorine. If the water is hazy or there's patches of algae or whatever, we SLAM Process. That's Shock Level And Maintain. If you had an infection, would you eat a bottle of antibiotics all at once, or take a couple a day twelve hours apart for a week? That's the difference between shock and SLAM.
Try to avoid powdered chlorine unless you know you need one of the byproducts. Dichlor and Trichlor are very acidic and will raise the CYA level. Cal-hypo will raise the CH level, which could lead to scale formation on the walls or on the SWG plates. More info here.