In a controlled environement, such as an indoor grow, flowering is induced within 7 - 14 days with most indicas and 14 - 21 days with most sativas once the photoperiod is shortened. So knowing this, it would take the same amount of days with the shorter photoperiod to induce flowering outdoors. Keep a light on them like your bro to keep them from flowering. However I'm not sure if a certain amount of lumens would need to be available to the plant in order to keep vegetating outdoors. After all, the sun is pretty freaking bright, so a 100w light might not be bright enough.....but not speaking from experience....

I don't know the correct answer to your question but this would make sense to me....

CGI::::::