Cannabis can be germinated and immediately placed into a 12/12 light cycle. I've heard of it being done, but also from what I've heard, it takes a lot of work to produce anything noticeable from it...and even then, it's a lot of work for a small amount of end-product.

A good rule of thumb is to let them vegetate until the plant shows pre-flowers, or until the nodes have started alternating. This indicates sexual maturity, and will certainly produce more abundant bud growth.