Depends on strain and technique(s) used, and how much headroom you've got. But if you don't top or take clones, (or otherwise injure the plant) 5 to 6 weeks is a good generalization. Once she starts alternating the internodes, she's ready any time you are.

Once in flowering, they'll start the stretch. About a week or so later you'll see the buds forming. Ya can't miss 'em, lol.