You have to replace the solution in the drop checker about every week or two, or they will be green almost all of the time, and a more pale green every day. Also, it is possible for a biofilm to build up on the water surface in the drop checker, which will interfere with the gas transport across that boundary - another reason to clean it and replace the solution every week.
I don't replace mine nearly that often, but I also don't pay any attention to the drop checker unless I am doing something different, like making a change in the way I introduce CO2, or changing the CO2 line in some way, or just trying something different. Once I know the level of CO2 is about right, I just leave the bubble rate alone, and ignore it, other than checking to be sure the CO2 tank isn't running low.