You'll be into the plateau soon and the rapid rise will stop. Could hang there 2-4 hours easily, may even drop a degree or two.
If it does seem to get done too quick, after checking the therm of accuracy (boiling water=212* and a small glass of ice chips=32*), make sure to check the temp in several different places. If the tip is stuck in a pocket of fat, the temp of the meat could be vastly different.
Cooking a 6.5 # butt should take a minimum of 6.5 hours, and two hours+ per # is not unusual.
Make certain you know the 101s on pork butt!
Hope this helps!