Here are some guidelines for cooking times & internal temperatures for various cuts of meat barbecued "low & slow" or grilled "hot & fast".
Please note that "guidelines" are just that, and your results may vary depending upon a number of factors, including: weight, shape/thickness, and internal temperature of the meat; amount of meat cooked; weather conditions; type and amount of fuel used; cooker temperature; and how many times you open the cooker or grill during the cooking process.
Also, remember that tough cuts of meat like brisket, pork butt, and ribs need to be cooked until they become tender, not just based on a certain amount of time at a certain cooker temperature. In the book How To Cook Meat, authors Chris Schlesinger and John Willoughby say that tough cuts must be "cooked through doneness to tenderness." In other words, you don't stop cooking a brisket when it reaches the internal "done" temperature we associate with tender cuts of beef like a Porterhouse steak or even a tri-tip roast. A brisket is not edible if cooked to 125°F or even 175°F. It needs to hit 180-205°F so that tough collagen can break down into soft gelatin, rendering a moist, tender brisket. You can read more about this transformation from tough to tender in the Brisket Selection & Preparation article.
Bottom line: It's best to determine the doneness of meat by measuring internal meat temperature using a good instant-read thermometer, not by how many hours the meat has cooked.
For Barbecued Meats
These are general guidelines for common cuts of meat barbecued in the Weber Smokey Mountain Cooker, assuming a 225-250°F cooker temperature.
Remember, allow large cuts of meat like whole turkey, brisket, and pork butt to rest after cooking. This allows moisture to redistribute and reabsorb into the meat. As a result, more of the juices will stay in the meat when you slice or pull it.
For Grilled Meats
These are general guidelines for meats grilled at high temperature—325-350°F or higher. When roasting a whole turkey, remove it from the cooker 5-10°F below the desired internal temperature; for a beef roast like prime rib, 10-15°F below the desired internal temp. Residual heat will cause the internal temp to rise during the resting period before carving. See Letting Meat Rest After Cooking for details.
Let me say this right up front—if you’ve come here looking for a pat answer, like, “Increase cooking time by xx% for each additional pound of meat cooked”—sorry, I can’t provide that information with any confidence.
Barbecuing meat is a function of time and temperature. You need to apply enough heat for a long enough period of time until the center or thickest part of the meat reaches your target internal temperature and becomes tender. For a 4 pound chicken, that might be 225-250°F for 4 hours. For a 6 pound pork butt, that might be 225-250°F for 12 hours.
Theoretically, it doesn't matter how many chickens or pork butts you're cooking. If you have a cooker big enough to hold 100 4-pound chickens with adequate air flow around each bird and can maintain a steady 225-250°F operating temperature, all 100 chickens will cook in 4 hours. Of course, it's going to take a lot more charcoal to cook 100 chickens than just one or two!
So increasing the amount of meat cooked is not about increasing cooking time—it's about starting with more hot charcoal so the cooker gets up to 225-250°F as quickly as possible after the meat is added, and it's about starting with more unlit charcoal so your cooker has enough fuel to operate at 225-250°F for the estimated cooking time.