Is 3 weeks of vegg enough?

Im gonna be growing 1 sour kush and 1 northern lights x big bud in a 2'x2'x5' box and i dont wanna vegg for to long cause i dont have alot if space so i was thinking about only vegging for 3 weeks then switching the lights to 12/12. So i wanted to know if 3 weeks off vegg is enough to get a good yeild? Im useing 150w hps for lighting. Thanks in advance

yeah that should work, if your'e not in a hurry though I'd top them after 3 weeks and wait another week or so until new tops develop
that'll increase your yield with just two plants.