edit: looks like I should type faster, lol. Seems Washougal covered the same ground. ^^^

I move 'em when there's space in my flowering shed. My sativa dom's go into flowering when shorter, indica doms get a tad more growth before the switch. Somewhere between 16 to 24 inches, depending on strain. By then you should be seeing the pre-flowers regardless of strain. :thumbsup:

A lot also depends on headroom in your flowering room, too. If headroom is limited, flowering a tad early may help by slight stunting. But if you put 'em on 12/12 too soon, they'll take a week or two longer to switch themselves to the flowering stage.

Chronological age isn't always a good indicator of being ready. Especially if using CFL's or fluoro tubes during the growth stage.