It depends on the strain and how long you vegged the plant... Usually takes a minimum of 5-7 days, but can take up to 2 weeks in some instances.
like IgnorantFool says, even up to two weeks for signs of pistles. Even longer for flowers to bloom and buds to stack.
I vegged for 36 days. I have had pistils for a while its just that I haven't noticed anything diff yet.
If you already had pre-flowers, you should notice some change soon. Indicas usually show signs of flowering in about a week, some sativas can take a lot longer... I have also found that if you are using cfls, it can take longer than using hps. Don't worry, the change will come...be patient...