In my experience, flowering can indeed be started at almost any time - but the longer you veg, the bigger the plant gets before starting to flower, so that will mean more tops and thus a higher yield. If you're only growing a single plant for example, it's usually a better idea to veg longer to get more out of the plant - SOG growers often induce flowering when a specific height is reached because there are more plants and less real-estate to grow in (smaller plants, but more overall plants per square foot).

Also though, some strains will mature (alternating nodes and preflowering) faster than others and thus can go into flowering at the time that this happens..or later as well. I've had my plants show maturity AND preflowers at 4 weeks veg, but I still keep them in veg until they reach the size I want (for yield reasons).

Typically, plants going into 12/12 will double in height, with most strains stretching during the first 2 - 2.5 weeks. Some will get 3x the height...usually Sativa dom strains... Peace!