When should I flower?

So I'm growing Nirvana's white widow

Coco/perlite/ flora nova series
T5 6400K veg/ 2700K flower
Distilled water 6.0 ph
I am looking for anything over an ounce..

When I grew this strain it was pretty easy to tell. The flowers plump up and develop a nice deep green. It just looks done. However to be sure, look at bud with a jewelers loupe. You want cloudy white trichromes. Clear means it's not ready and amber means it's over ripe. You'll always have a bit of everything but it's the overall ratio that you look at. It's still an art and take a few grows to perfect. So make a personal judgment and adjust next time.

I think the question was about when to start 12/12, not when to harvest. If all you want is an oz. you can start flowering anytime.

Thanks guys.. yeah the question was when to flower not to harvest

You're welcome. Some people start their plants at 12/12 and keep them that way until harvest. Smaller yields but more frequent harvests. It would be interesting to find out which system yields more in a perpetual grow setup.