Enhanced co² is good from the start of vegetation cycle to the finish of flower cycle. Including up to the day you harvest.

General air is about 300ppm so you need to add about 1500ppm of CO² and have a sealed room to hold in the gas so the plants can absorb it. The idea is to hold the level of about 1800ppm from lights on to lights off. You only exchange the air in the room a few times a day then build it back up again.

If you don't have a controller to monitor the level then try the old method but it's just a guess. Just use a timer and hope you have it right. The problem with using a timer is you are guessing. You have no way of knowing how much leaks out and how much the plants use before your next refill of the room.