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Check a cookbook. Smoker, oven, doesn't matter. By all means though, go by internal temp. But according to Better Homes and Garden Cookbook, a #13 turkey @ 325* should take around 3 to 3 3/4 hrs. Again I stress, go by internal temps.
Yea - What Crewdawg said. I did a 13 pounder yesterday and had some trouble keeping a steady 325°F because of wind and 33°F ambient air temps. It took me about 4 hours.