I'd say 4 weeks veg is the minimum you'd want to veg for. I've heard if you try to flower at say 2 weeks the plant still has to stay in veg mode till around 4 weeks when it reaches maturity before it can start to flower so all your really doing is derpriving the plant of light which could be used for additional veg growth. Do a search for logs that have had short veg periods and see what they're results are like.