There isn't really a maximum. Or if there is, you don't want to get anywhere near it.
The brightness really depends most on how far the light is from any surfaces.
If you have two lights, @A and @B, and @A is twice the distance from a surface, then it will need to be FOUR times as bright to light the surface the same amount:
@A- - - - - - - - - - - - |\\ surface
@B- - - - - - |\\
Sky brightness works more like a texture light, but it also doesn't fall off with distance. Every sky brush surface will provide some light, like a tex light. And the direct sky light is just as bright at any distance.