I always think that late January/February is a good time. Maybe because that's when I did it.
I do genuinely believe it has things in its favour though - late enough for the Christmas backlog to have been cleared, but still in that "new year" mindset of looking around for the next big thing, and shortly before the run-up to the London Book Fair, which can be a good time to sell. I definitely think the timing speeded things up for me.
Of course, this could all be bollocks. Still, you've got to keep some kind of faith eh?