Determining Distance of Indirect Lighting Influence

[rleas]

Anyone have a good formula for determining the maximum distance of indirect influence from a given light?

For instance, let's say we exclude texture and color variations by saying we have a white directional light with an intensity of 1 hitting an object with a diffuse color of pure white.

In V-Ray, does the quadratic falloff rate apply for indirect? And if so, what's my best means of determining the effective distance of influence?

Thanks.