when do i use co2

I want to use co2. when do i use it, in the veg stage, during floering or all the way through?

Some of this depends on your funds, and how are you introducing co2.
At any point in a plants life, it can benefit from co2 usage. However, alot of people only use it through the flowering stage, as the desire is for bigger buds. I used it as early as 4 days old myself.
Peace