On average, it takes me about 2 months for the first draft.
With that said, every book is a little bit different, and some take more or less time than others. The Grant Wolves books often take a little more time because they trend longer, with most clocking in the 80-90k range, and because they require a bit more coordination with Joey and Chris both vying for page time.
I am fully capable of writing 50,000 words in 30 days (thanks, NaNoWriMo) but I find that my quality is better—and the editing process goes more smoothly—when I set a more relaxed pace.