smoke and heat escape every time you peek into the grill, so add 15 minutes to cooking time for each peek (more if you are smoking in cold weather).

Weber has times and methods.
http://www.weber.com/bbq/pub/recipe/g...
The times they list are for UNstuffed.

Cooking Times for Unstuffed Turkeys
10-11 lbs. = 1-3/4 to 2-1/2 hrs.
12-14 lbs. = 2-1/4 to 3 hrs.
15-17 lbs. = 2-3/4 to 3-3/4 hrs.
18-22 lbs. = 3-1/2 to 4 hrs.

P.S. Even though I don't drink, none of my friends have ever complained about someone bringing too many six packs.