When cooking a brisket at 250, the general rule of thumb is that 1 pound of meat equals 1 hour of cook time. More precisely, smoke until the internal temperature of the meat reaches 190 to 200 degrees F.
In general, smoke a pork shoulder for 75 minutes per pound at 250 degrees Fahrenheit. But the actual cook time depends on internal temperature. A safe internal temperature is 185˚F, however, the ideal internal temp for tenderness is 190-200˚F.