What lamp?
150 watt, 175 watt, 250 watt, 400 watt, 600 watt, or 1000 watt?
It depends on LOTS and LOTS of things. The VERY BASIC and rough estimate is 1 gram per watt of lighting but I have seen more and a whole lot less than that. Completely depends on the strain, skill of the grower, conditions, etc., etc., etc.
Hard to answer question bro, you need to be A LOT more specific.