It's not as detailed as Avian's posts (you kids with your flashy charts ), but here are how many army books/codexes we've had per year since 2001 (not counting campaign books etc):

2001: 3 Fantasy, 2 40K, 5 TOTAL
2002: 3 Fantasy, 2 40K, 5 TOTAL
2003: 3 Fantasy, 2 40K, 5 TOTAL
2004: 1 Fantasy, 2 40K, 3 TOTAL
2005: 2 Fantasy, 2 40K, 4 TOTAL
2006: 2 Fantasy, 2 40K, 4 TOTAL
2007: 2 Fantasy, 2 40K, 4 TOTAL
2008: 4 Fantasy, 3 40K, 7 TOTAL
2009: 2 Fantasy, 2 40K, 4 TOTAL
2010: 1 Fantasy, 3 40K, 4 TOTAL
2011: 3 Fantasy, 2 40K, 5 TOTAL
2012: 2 Fantasy, 1 40K, 3 TOTAL

So, most years we get 4 or 5 army books/codexes. Sometimes a bit less, and in 2008 a hefty 7 (aided by having the double release of Daemons), but that's usually it. And of those, typically there will be 2 or 3 of each, but this can go down to just 1 book for one system. As usual, any estimates that run along the lines of "We'll get this book in January, then this one in February, this one in March, that leaves a space for this one in April..." are over-optimistic.

Personally, I think we'll see quite a bit next year, for three reasons. Firstly, this year has been quite a quiet one for books, with only three out. Second, we're starting right away with one book (Dark Angels) and have plenty more being talked about in rumours. Third, we're supposed to be getting Daemons, presumably another double release. I've seen talk of Daemons, High Elves, Lizardmen, Warriors of Chaos, Dark Elves, Dwarfs...it's pretty safe to say we won't be seeing all of those, and that certainly wouldn't leave any room for Bretonnians or Wood Elves unless they come out from nowhere. But the most-talked-about Daemons, Lizardmen, High Elves and Warriors of Chaos are all possible, even if that's quite generous.