I tend to do about a 14-lb turkey around 6 hours over a mix of wood and charcoal (both natural lump and some briquettes).
BTW I recommend putting rub/herbs plus a multi-punctured lemon or lime and/or an apple in the cavity and run the spit through them. It keeps the bird from bouncing around and you get all that extra flavor and humidity working for you.
Here's the 16.5 lb one I cooked on Saturday in Albuquerque that took about 7 hours over an open fire pit over a mix of peach, red oak and mixed charcoal:
It was the first time on the outside I had used a Cajun rub and nothing else. No brining, no oil, no nothing. It was surprising how far the (delicious) flavor carried into it as it really didn't need any salt. Nice and moist, too, despite the fact a few spots popped open from the heat.
Happy turkey cooking everyone!