I think most of us will be within the same range. Something to consider is what the client wants. I'm working on a project now where we're coming up with multiple paint ideas, each "Sketch" is running an average of an hour or so, and I currently have 7 of them. Of course theyre the same two views, each having master line drawings, that took about an hour to do. To point this out, these "Sketches" are not perfect, they are what I call quick and dirty sketches, and theyre used to get the point across without wasting the clients time and money.
When we reach a final design, I'll work up another set of master line drawings, where I'll take more time into perspective, proportion, elipses, and reflection mapping, and come up with the final rendering. At this point in my life I still prefer old skool techniques using marker, pastel/airbrush, pencil, and gouache. The final rendering is very important, it's the communication device, hence the detail, and with detail comes time, a whole lot of time. I try to set a goal between 10-24, some may take longer, some may take less.
Speed comes with practice, especially if you've done it before, like same colored cars, same medium used to come up with the work. Canson renderings take me forever, becuase I don't render on Canson as much as I do white paper. The same goes for digital rendering, I'm not used to it. Ben's got the advantage there he does have that undo button!
Hope this helps!