after a week on 12/12 you will see white hairs or pistals coming out of the internodes, should be budding up by 2 weeks later.
I have heard you can put plants in total darkness for 36 hours before flowering schedule to speed up flowering it is sposed to reset their hormones or something
I would say when you switch to 12/12 a more realistic time would be about 2 weeks+ before you start to see pistils and males will show them selves a lot sooner.
I usually start counting flowering days from when I first see pistils, as counting flowering days from when you switch to 12/12 you could effectivley miss the last 2 weeks of flowering and this is when the plant can put up to 25% of it's weight on.