how long to sex plants under 24 hr light

I am new to growing indoors but have grown outdoors in Hawaii. I have a small closet and will use cfl lights. I will cover walls in tin foil and use plenty of light. I plan to keep lights on 24 hours during foliage stage. From experience growing outdoors the shorter photoperiod stimulated flowering. With the 24 hour light how long should I expect to have to wait to sex the plants? With the limited space I want to get the excess plants out as soon as possible.
Thanks,
RJay

when they have 6 nodes switch to 12/12