Do I need to just keep adding bleach 4 or more gallons at a time?
You must test before you add ('til you figure out how much Cl you're consuming).

If your water is not yet as sparkling as you'd like, Test and add enough to get you back up to the 25ppm range.

And how long do I wait before I can get an accurate test?
I don't understand this question...please clarify.