Assuming mental ray renderer.
Go to the Render Setup dialog, Indirect Illumination tab, and check Use Falloff (Limits Ray Distance).
In the illustration, the left render is with the Use Falloff not active.
The middle render is with it active and the Start at 0' and Stop at 1'
The right render is with it active and the Start at 0' and the Stop at 10'
The Box is 2'x2'x10'
Is that what you need?