your going to get ripped apart for asking such a simple question without just looking yourself lol.....ill be nice and tell you what you want to know..in general most strains available cater toward a short grow time and will flower in about 9 weeks from the day you changed to 12/12, some hybrids and sativas can take up to 14 weeks....what strain are you growing? bagseed? well if you dont know the strain you just wait until at least 8 weeks as thats the shortest flowering time aside from only 2 strains ive ever heard of which take 6-7 weeks..at 8 weeks you start to watch it for signs that its done crystals cloud up, more orange hairs, the buds will swell and ripen..you can come back on and ask people if its done thats a much more acceptable question to ask lol