how long for drop-checker to change?

It takes about 2 hours for a drop checker to get close to a stabilized condition. So, if the bubble rate on the CO2 isn't constant, the drop checker will never catch up and give a correct reading.

also it depends on the tank size. I was having regulator problems a month or so ago, got a new needle valve and now with the consistant bublble rate it took a couple days for the tank (55g) to get back up to 30ppm. Now i'm there again and set to go.