You can flower whenever you want, but less vegg. time = less yeild.

It all depends on the strain as well. Indicas tend to go short and stumpy, so their size at the end of vegg. time will only double, with proper lighting and conditions, as where sativa dom. strains, they will at least double, but most likely will triple in size.

I grew a WW once, mostly Indica, started flowering at 12 inches (1ft) in height and ending height was only 2ft.

I have a Jack Herer cross right now, 80% sativa I believe, and I flowered when it was about 10 inches in height, and the bastard is at about 3ft right now.

So it all depends on the strain. Indicas = small/thick-Sativas = big/lanky