At temps of 200 to 250, be prepared for what is called "the stall". The internal temp of the meat will stop going up, and in fact, might even start dropping a few degrees. When this happens don't worry and think that your thermometer must have broken, or that you've entered the Twilight Zone as the laws of physics are being broken. What is happening is that the meat is undergoing evaporative cooling, kind of like we do when we sweat.
There are three things (or combination thereof) that you can do when you hit the stall.
1. Wrap with foil.
2. Crank up the heat.
3. Kick back, open another beer and just let it ride.
Myk, yes, 1.5 per lb is a good time guestimate at 250. But, note that this is for 250. If you drop down to 225, it goes up to about 2hrs.