In What does the Roughness setting on a diffuse shader do? Ibalaszs states that he divides the glossy roughness by 3 to get the diffuse roughness. He then states that this is also just a thumb rule he somehow eyeballed, but also makes sense as most materials have a diffuse roughness of up to 0.5 while most are topping of at 0.3. So apart from that does anyone know how to actually use the diffuse roughness value and do you even use it at all? I did some render test with suzanne at different diffuse roughness level:
One thing I noticed that with high diffuse roughness values the shader almost starts to look like velvet.