Some of mine started to show sex a couple weeks ago and a few I still can't tell. Wierd how that works but I think in my case it is because of shade in the late afternoon, gets darker sooner.
Also I have five different strains going so they also may have much different flowering times. That's what I get for using free seeds, I also had a bunch of males and not so many females. Learned my leason, back to clones.