A 12pt font in word, double-spaced, gives you approximately 250 words per page. I don't know how you'd translate that to a Pathfinder module, though I suppose if Adobe has a word count feature and you had a pdf version of an adventure, that might help.
Of course, by the time you did all that, someone official will probably have posted an answer for you. =)
I would like to know as well. (I would guess someplace around 20 thousand words, the 16 page 2e AD&D module I recently wrote had roughly 10 thousand words, though of course I was using a different formatting style than pathfinder modules use).
It comes out to around 20k words or so, I do believe.
This.
It actually depends on the number of maps and art mostly. Fellnight Queen originally had room for 22K...but then, some changes in layout design forced a bit of trimming to get it back down to 20K. Of course, the fact that I wrote enough to cross 24K didn't help. Needlesss to say, Sean had to leave a few things on the cutting room floor. But he did a great job preserving the important stuff.
But, in general, you should plan on 20K. There's some likely wiggle room to allow up to 21-22K, but that's really it.