When you start to flower, 12/12, they willstretch to about twice the height they are when start. they are mature and redy to flower when the branches at the nodes start to stagger as opposed to being directly across from one another on the main stem. I am a new grower too but this is what I have learned by reading and by seeing on my first grow which is on day 38 (10 days of flower). mine have already almost doubled in size just in the 10 days since i switched to 12/12. there are pics in my album if ya wanna check em. there are some at 25 days veg and i just put one up of em of day 38....big difference.