Here's a test I left running over the weekend to try out my fill lighting setup with fake stones. I finally scaled things down correctly, although the displacement on the rocks doubled their size.. but I left them big to get some decent shadow detail. Detail 2, AA 4, no GI, original render 2000x1000.
Full size render here
http://www.path.unimelb.edu.au/~bernardk/tgdemo/rocky_valley2.jpg (1.2Mb).
The shadows were still a bit dark so there's some post processing in Photoshop to bring out some extra detail,(Shadows lightened 25%) but the colour and detail held up to the adjustment. I added an additional blue fill light from below (no shadows) to provide a bit of a kick under overhangs. I'm guessing the missing shadows of the rocks will be fixed with GI on rocks.
5 Lights, but only one casts shadows so there is not too much impact on render time.
Now to tweak the levels....