Hi Chris,
Many different versions of essentially the same method to do butts. Here's mine. The day before the smoke, paint your butts with plain yellow mustard and then apply your favorite rub. Tightly wrap in plastic wrap and back into the frig overnight. Day of smoke, take butts out of frig for at least one hour prior to smoke time. Preheat smoker to 250-260 degrees. Don't worry, when you open the door to your smoker and place 15lbs of cold meat in there, the temp in the box is going to drop like a stone. Apply smoke of your choice for 4 hours. Being from KY my choice is hickory! You want to be smoking/cooking at 200-225. As the temp comes back up in the box, you may have to adjust your settings. After the smoke period, the Bradley is now just an oven. For better temp control, I transfer butts to my house oven set at 225 at this point. Some folks wrap in foil, some boat them with apple juice. I don't do either. They go on a rack in a roasting pan until their IT hits around 190 degrees. It is is then ready to pull. If this is your first time doing butts, don't get alarmed when the IT stalls for several hours at around 150-160. That's what you want them to do, the collagen and connective tissue is breaking down. You asked how long it will take. each butt can be different but if I were you and I wanted to serve around 5:00, I would start around 5:00 the day before. If they get finished early, you can always FTC them. I have had them take 24-26 hours before and I have had them finish in 15-16 hours.
Others will come along shortly and give you better advice then mine.
Enjoy!
KyNola