I did mine for thanksgiving on my weber kettle grill. Took right at 3 hours. I used Kingsford mesquite charcoal. Start with 25 lit pieces per side, add 8 pieces per side every hour to maintain heat. Works perfectly and the turkey is AWESOME!
11 Minutes per pound unstuffed, 13 minutes stuffed.
Here is a great tutorial on the method.
LinkWe did a 14 pounder in IIRC between 2.5 and 3 hours and it was still juicy and tender, be sure to put a pan in the center to catch drippings for gravy.