ARC=Advance Reader's Copy. RJ books don't do that, for one.
Also, RJ takes so long to write his books that I'm sure the art department has plenty of time to work on what they need to do prior to his turning in the final manuscript. Artists need anywhere from a month to several months to do cover art. I'm not sure how long Darryl Sweet takes.
The six weeks that Tor is talking about is just the time it takes to usher the manuscript through a copyeditor, a proofreader, and off to be printed. Everything else that needs to be done will have been prepared beforehand.
However, RJ is an anomaly. For one, his wife is his editor, so he really does have a longer editing process than that six weeks implies. For another, most editors and art departments need a finished (draft) manuscript in hand so they know what the book is about. I also have a feeling that when RJ turns his manuscript in, his editor in the Tor offices (the one who isn't his wife) drops everything and works only on his books, which if every editor did that for every book we'd have the most erratic schedule on the planet.
So no, six weeks is definitely not typical, and that number probably (I don't work at Tor so I can't say for sure), *probably* doesn't reflect the time people in the office spend on the things like cover copy, catalog copy, sales conferences, art, design, etc. Plus, being a series, they do have some of those as a given, since they're not going to change the design in the middle of the series.
I'm sure it took a *lot* longer for Eye of the World to be published than it takes for book 13.