You won't generally see pre-flowers until you get to at least the 6th internode. So that's at least 6 proper sets of leaf's. You are probably talking 3 weeks to a month to get to that stage.

You can flower from seedling by putting the lights straight onto 12/12 but the plant will only actually start to flower once it gets to a particular level of maturity. So even though you put the lights on 12/12 the plant won't flower until its mature enough which again is going to be at least 3 weeks to a month.