If the lighting is really bad that can happen. If the test was for that many characters you might want to check that somehow for the 5 hours the rendering didn't jump to cpu rendering.
Anyways, nice scene, 8 characters is already a lot. What graphics card do you have? Did you optimize the scene at all? (in the sense of lowering poly count or the texture resolution) Also, if you add clothes, since they have textures as well, some also maybe a lot of polygons, you might go over the VRAM limit already, have to pay attention to that as well.