seems to me it depends on strain, I have had some show pre-flowers once you reach the 4th or 5th node, some don't show till you switch your lighting, and then it still takes a week or two. If you can upload some pics we can have a look.
no easy way my friend
earliest is like go4 says "it's about the light cycle"
about 2 weeks after 12/12 is started usually give or take a few days
welcome to 420mag
