Different growers do different things. Some people cut off co2 completely for the last 2-3 weeks. Some drop it from the high levels in mid flower down to around 600ppm for the last 2-3 weeks (which is what I would reccommend). Some don't drop co2 too much (like 1600ppm to 1400ppm and then 1200ppm before chop) although critics say this restricts the terpentines and cannabinoids that produce the beautiful smells, flavors, and colors. IMO max at 1600ppm you can go higher or lower though. Good Luck.