An indoor setup with a photometric light and a skylight.

Set up an interior scene to scale, in other words, a ceiling with a believable height.

Illuminate with one photometric area light.

Logarithmic under Rendering/Advanced lighting ...

Now, turn on mental ray.

Enable default photons ... look at the colors bleeding into the gray walls:

Note: you can get real specific about the max sampling radius this way. Take a tape measure across the camera view.

Take note of the length and divide by 10:

Enter that approximate value here:

Increase the average GI photon per light from 10,000 to 50-100,000. Still dark but the photons are evening out if you look closely.

Add a standard skylight [not IES sky] and turn on FG [Final Gather]

You can also optionally set the radius up to equal the GI max sampling radius [I converted to generic units here] then set the min radius to 10% of that.

Turning off preview is a fast way to simply evaluate the light.
The result is block-like but fast.

I am here turning the skylight down and increasing the FG samples.

Turn preview back on.

OK we can do one more thing. Renderer tab: 1, 16, and Mitchell. This will fix artifacts on the edges of the right hand shelf:

Go for broke, 1,000 FG. The result looks great, but it is very slow: 25 minutes.

You can have some fun and apply self-illuminated materials to objects.

[Tip – put a fresnel falloff in the self-illumination slot]