Well I find that when I used CO² not only was my yield greater but the total time to finish was reduced by 2 weeks. You do need to have a proper sealed room and control the environment but YES it will reduce your over all time by 10-14 day's or more in some strains.

The only way to tell when you are done is by viewing the trichomes with a 30x pocket lighted microscope.

Plants don't read calendars; follow the trichomes to know for sure.