Exactly when do I swtich from 18/6 to 12/12?

I have a few indoor plants that I started from seeds. They are just over 2 months old now and they are starting to develop their sex organs. This is the first time I'm growing these plants, and I'm unsure when the "BEST" time is to switch from Metal Halide 18/6 cycle to High pressure sodium 12/12. Should I wait until the sex is clearly shown (or even later), or should I switch the cycle now and pull out the males once the sex is established?
Any help is much appreciated!

wait until its clearly shown

just pay close attention and remove the males as soon as your positive they are, to avoid pollination

depends on how big you want them to be. they will double or triple in size before they are done. 2 month in veg. you are probly ready to switch to 12/12 now.
if they are getting preflowers then it wont take long for them to really show sex, then pull the males.

I always have to add this, if its a really kickass variety, you might want to clone them all and choose a mother plant out of those clones after you harvest, based on what ones grew/smoke well. I made the mistake not to do this when I was new and regretted it. Good luck!