Yep, follow Chile's advice. I do several turkeys a year, most between 16-20 lbs. 18 lbs is my favorite weight. I've had them finish is as little as 2 hrs 45 minutes, to a long as 4 hrs 30 minutes. Temps are never exact in a smoker, but here are my results for the three I've done this year.
18.98 lbs, average chamber temp 354F, spatched, done in 4 hrs, 5 minutes.
16.29 lbs, " " " 335F, left whole, done in 3 hrs, 15 minutes.
16.06 lbs, " " " 307F, left whole, done in 3 hrs, 35 minutes. I didn't add enough hot charcoals to my cold ones so the chamber never got up to where I wanted it, but it was still delicious.