Consider mother natures natural growing season for cannabis. You should be able to grow outdoors and maintain vegetative growth from about May 21st to August 21st, thats three months of about 15 +/- hours of natural light. If you're growing 24/7 then you should factor your growth rate at 33% higher then mother nature, if mother nature gives you 90 days of veg growth subtract 33% from that and you should cut it to about 60 days for the same growth rate. HOWEVER what you can't factor is the power of the sun vs. indoor lighting.

The ideal situation would be if you could give them their normal natural sun during the day then put them under lights at night. I tried this by moving them by hand back and forth but after a few weeks I said fuck it!