MagpieTear

I have the little clear plastic bubble CO2 level checker but have a silly question not covered in the instructions. Are you supposed to dump the test solution out after you get your reading, or just leave it in there for a while to continuously monitor the levels? And if the latter, how long? a few days? A week? I'm guessing this is common knowledge among the experienced Aquagardeners, because I'm not finding an easy answer anywhere, and I'm not ashamed to admit I'm clueless here...