The rough estimate usually used is 1-1/2 to 2 hours per pound per piece (so 9-12 hours), but there can be variation depending on cooking temp, and each butt is a little different. Butts have a lot of intramuscular fat, so brining isn't really necessary or recommended-- there's not much chance of drying them out. An injected marinade, however, can get some good flavors in there. Butts are also very forgiving, so if you cook at 225°, or 250°, or at some points even a little higher, it won't appreciably affect the outcome. Go by your thermometer, but also look for the bone to start protuding just a little, and test for tenderness with a fork when it does.