The VOIDRUNNER'S CODEX is coming! Explore new worlds, fight oppressive empires, fend off fearsome aliens, and wield deadly psionics with this comprehensive boxed set expansion for 5E and A5E!
I would think it depends upon the book. One of the campaign settings probably takes longer than a power book. Or a mega-book like the Pathfinder Core takes years. I also wouldn't be surprised if there is a kind of "gestation" for many books, like monster books for example, where the designers are told "think up 30 monsters each over the next six months" rather than "think up 150 monsters as a team over the next month."
Given previous statements and incidentally collected data, I believe it's anywhere from 10 to 18 months, depending on the size of the project. Mearls mentioned one developer who had been working on D&D Essentials since September of 2009, as one recent example. In other statements, Dave Noonan has said before that most design work begins about a year out, and development work a few months after that.
I'm betting they're working on products for Q3 2011 as we speak.
IIRC, WOTC announced Dark Sun at GenCon last year, which suggests there's at least a year's lead time. And IIRC, GenCon announced a Ravenloft RPG at this year's GenCon, and that's slated for a fall 2011 release, also about a year's lead time.
Yes, though it's fairly rare. I remember that Dragon Magic for 3.5 was fast-tracked and produced in about 1/2 the normal time. This was something stated by one of the designers (I forget who) when it was discussed in panel when revealed.
From my own limited experience, it appears to take between 8-12 months minimum to write a decent 120+ page book for a single author.