Jason - all other things being equal, that sounds about right - 9 - 10 hours. Is it a bone in pork butt? I've never cooked one that has been boned, but some folks report that those may take a little longer. They feel that the bone conducts heat into the middle of the butt and helps the cooking time - who knows?
Of course, some folks here, when you ask "How long does it take?" reply "until its ready! They are right, of course.
I always make sure I give myself at least a couple of hours to spare time before its time to pull and serve the butt.
Just be sure to resist the temptation to try to push the butt through the plateau by upping the heat. In my experience, that yields a finished product that doesn't have the tenderness or texture of a butt that is allowed to work itself through the plateau at a steady temperature.
Of course, there are those who say that you can cooking a butt just as well at 300 for a lot less time, but I'm not of that opinion just yet.
Have fun with it - My wife is from Houston and we were married there. I sure do miss the excellent Tex-Mex, but I don't miss the traffic!
Pat