I'm reluctant to disagree with the princeofpork. But.... I go for an internal temp of 190. I think it CAN be cooked too long. There's falling apart tender and then there's loss of flavor. IMO, of course.
You won't need as much time - two to three hours, depending on how thick the shoulder is, and whether or not you sear it first. You can cut it in half to decrease the oven time even more. Start checking at 90 minutes.
I cook 4-6 pound boneless butt at 225-250 degrees for at least eight hours (any size) and then start using a thermometer. For bone-in, disregard the thermometer and look for the bone to pull right out.