Depending on the parameters, renders can take 6, 8, 10 or more hours. The thing about unbiased renderers is that you pretty much just set your lights where they would be in reality and just let it go and you'll get a great looking result. With other methods, you have to fiddle at placing lights, rendering and then re working them over and over until the scene looks right.

For me, it's easier to set it up and then let it run over night. When I get up, it's usually looks great!