When cooking pork shoulder in a smoker at 275 degrees F you cook the pork should an hour for each 1 lb of pork shoulder.
So if you have a 5 lb pork shoulder cooking in the smoker at 275 F you would cook it for 5 hours and so on.
Take the weight of the pork shoulder you have and then cook it an hour for each lb.
Another example is if your pork shoulder is 10 lbs you'd cook the 10 lb pork shoulder for 10 hours in the smoker.
If you want the 10 lb pork shoulder to be smoked more then you can cook it 2 hours per 1 lb of pork shoulder which would be 20 hours or even 16 minutes.
Usually though 10 hours or even 8 hours is usually enough cooking time in the smoker for even a 10 lb pork shoulder.