The decision of how long to veg is directly related to the size available and the lights that plant will have access to. As a rule, plants tend to increase in height 200% within the first three weeks after flowering. It has been my experience that weed vegged longer is more potent and has bigger bugs, but this may be directly related to size.
Most people dont veg for too long, i vegged once for 17 days only after germination. If you look at my last post, those photos I put up are of plants from seeds that vegged for 7 weeks.