depends on strain. I think it as to do with maturity. My current grow would not should sex until it go to the 7th node. Im pretty new to growing myself. Just my opinion.
usually takes mine 3-7 days after i switch to 12/12. they will not show "buds" in this time, but the pistils will show.but i have had to wait up to 3 weeks to a month for some young plants. to see actual buds it takes like 3-4 weeks usually if they switch to flowering in a decent time.
your looking at about 2 months for a good size from sprout atleast and my plant is 28 inches tall and took 3 weeks of 12/12 just to sex and now its on its way . well as of 7-10-09 is when the 1st pistils came out any many more are coming out . and im only 2 days in of pistils bro .
heres my link to my page . its a sativa hybrid so it grow random leaves. it went from 2,3,5,7,9,10,11,12 and back to 5,7 as it grew and its been fim`d at 7 inches twice and toped at 20 inches and still going
I know im sounding like a parrot since its been said but .... lol
To flower your plant needs to be sexually mature. Most plant's begin to show first signs of pre-sex @ 20 days and upwards to close to 2 months for others. So it all depends on the genetics.