I've seen grows that produced 5 lbs per 1,000 watt, but the trick is they spend 3-4 months in veg and another 2 1/2 months in flower. So 6 months to produce that 5 lbs.
After 4 months in veg the plant wont fill in any more space than say 12 plants vegged for 1 month.
When you are talking length of veg there comes a limit where it is counterproductive to veg anymore.
After all a 1000 W does not cover that big an area effectively, there is only so much plant that you can fit under it.
And you can't fit 5 pounds of plant under a 1 000 W light, you need space and the sun to do that.
About the most you can get under one 1 000 W ligth is a pound of buds, but that is only with a high yielding strain, and the norm is less.
Does not matter how long you veg them for after a certain point, they will just end up too big remember you can only harvest the top 8 inches of so of buds, as the light does not penetrate sufficiently to anything lower than that, so no point in growing 8 foot plants indoors when you can only harvest the top 8 inches effectively.