The longer you can leave them in veg the more budsites you will be developing and therefore the greater your yield potential. Try to let them veg for at least 4 weeks minimum, 6 weeks is better, 8 is great. The plant will flower after you switch the light cycle at pretty much any point, but the best way to tell when the plant is ready is when it starts growing alternating nodes -- in other words, in the beginning at each node are two leaf-sets on opposite sides of the stem, but at some point (after 6 weeks or so usually) the nodes will start alternating, with only one leaf-set and then the next leaf-set on the other side of the stem at the next node.