The length of time depends on your smoker temperature. When smoking at 250 degrees, allow 1 hour of cook time for every 1 pound of brisket. A 2-pound brisket only takes around 2 hours in the smoker.
The general rule of thumb is that every pound of brisket needs one hour at 250 degrees Fahrenheit. Therefore, 15 hours should be enough time for a 15-pound brisket, and a 2-pound brisket would take a couple of hours. But that’s not the whole story…
At 225 degrees and 1-1/2 to 2 hours a pound, a 3-pound brisket needs between 4-1/2 and 6 hours to cook. At 250 it would take about 3 hours.
When cooking a brisket at 250 degrees, one pound of meat equals one hour of cook time. By that measure, 10 pounds of brisket takes about 10 hours in the smoker.