average time is hard to say. anywhere from 8 - 15 weeks. example being blueberry can flower for 8 - 10 weeks and be sweet as hell, but then something like arjan's haze #1 can take up to 15 if you let it. i did a AH1 grow a while back and that shit flowers FOREVER but yields like a hot, sassy bitch.
i would always stay on the safe side and flower for at least eight weeks. get a pocket microscope and check out those trichomes. the worst feeling in the world is to harvest to early because of impatience and then feel like you wrecked some decent weed. this whole process takes time, so yeah, give it 8-10 weeks to be totally safe.