If your using CFL's or tubes you should have already changed the lights because their limited lumens are not adequate to support a large, outdoor sized plant. Seemorebuds suggests in his book that if you use CFLs you should flower the plants after about 17 days from germination. If your useing HPS however you can support many more, larger plants, and thus have a higher yield if you hang in there maybe a week or two more. There is really no maximum amount of time you can Veg a plant, it really all revolves around your lighting setup.