To quote Ms. Haze:
"Almost all growers seems to agree that you can get increased growth and bud production by running CO2 during the first 2-3 weeks of flowering.
After flowering is fully underway, some growers believe that CO2 is less effective, and it’s true that so far no one has been able to prove that CO2 can directly improve bud production beyond the first few weeks of the flowering stage.
However, a lot of growers feel from personal experience that running CO2 does increase bud production. Some growers recommend that you keep running CO2 all the way until 2 weeks before harvest."
I've read if you run co2 too long into flower it can prevent ripening. It also may prevent the buds from hardening.