You know it depends on a lot of different things. Number of subjects is big. Getting the textures just right, background, lighting, poses and camera angle. But 3-6 hrs is the low end 12-24hrs on the high end.
I like to streamline the processes as much as possible too. Like I made a macro/script that will automatically enhance and create the frame. Something that would normally take 5-10 minutes now takes 10-15 seconds. I try to save poses and textures as much as I can too. So once I have a 3d scene saved and I need to tweek it it usually does not take too long. And my machines are tuned to high end 3d rendering. A rendering even at over 10,000 pixels across takes minutes (usually, depending on elements....subjects, lights reflective surfaces and transparencies....) But I have done scenes with literally millions of polygons and many transparent objects that took hours. The longest actual render time I think must have been over a day just for the computer to render out. But that was a while ago, on an older system.