I work in publishing. Here's the break down of a books schedule.
Lets work backwards...
Time from delivery of files to printer for the book to hit the shelves = 3 months.
This is mostly due to delivering the full-color book from China where most full-color printing is done now (watch out for those red pages). Book launch in May for PHB1 means they have to get the files to the printer no later than February 2008.
Finalized book manuscript goes through layout and design = 4 months.
Gathering all the images and making sure they're all in the right places. Determining the heads, subheads, sub-subheads, etc is a long process. With constant tweaking and hunting for widows and orphans. And there's the infamous backwards art, etc.
This means the text (rules, fluff, everything) has to be set in stone (very minor copyediting can be done at this point, but emphasis on minor, every time you change a line length, you have to reset everything that comes after it in a chapter, i.e. you piss off the typesetter, and that's bad) no later than early October 2007 to hit that launch date above.
That's what, 6 weeks of playtesting, at most, if you keep to schedule and if you want the book out on time.