read this http://forum.grasscity.com/absolute-beginners/101130-sexing.html to quote that thread "When the plant is 6 to 8 weeks old it develops pre-flowers" Thats when its mature. You can flower from anytime but it is better to wait until pre-flowers show otherwise you can have a hermaphrodite.
if they are grown well, you can see pre-flowers 3-4 weeks from sprout. but you can start flowering before they pre-flower, it'll just take then a little longer to adjust to the new light cycle which is no big deal. i flower when my plant has reached a size that is about 1/2 or 1/3 the final plant size i want. i typically have room for a 5ft tall mature plant, so i'll start flowering when the plant is about 2 1/2 ft tall. the longer your veg the bigger yield you'll get, but most indoor growers are limited by the size of their grow area. bottom line: just start when your plant has reached about 1/2 the final size you want it to. as i said before, this is about 4-5 weeks of veg under a MH bulb. good luck.