You can technically flower any time you want, at least after you've got two nodes (I think), but you won't get very good results if you do so before a month of solid growth has passed. You want the plants to be healthy and stable with lots of budsites, and the less you veg the less of those there will be. If you're really concerned about space I'd recommend LST or ScROG, rather than sacrificing both quality and quantity by flowering too soon.