Two respected agents blogged recently on the time it takes for a book to go from submission to acceptance.
Jessica Faust of Bookends talks about it in her March 11th blog.
Nathan Bransford of Curtis Brown talks about it in his March 6th blog.
Please note that both talk about books submitted by literary agencies. A book submitted by the author could take much, much longer.
NOTE: I'll be a guest at Stellarcon/Deep South Con (High Point, NC) this weekend. If you're there, come and say "hey." I'll be the matronly hobbit with the guest badge.