The problem is that each piece of meat has a different structure and takes a different time to get done. Also, each smoker has its own thermal characteristics, even at the same temperatures, so the time varies once again. That is why you will hear "it is done when it is done".
That being said, a very rough rule of thumb is 1 1/2 hours per pound for brisket and pork butts when you cook low and slow.
Jim - Another transplanted Texan
Former KCBS CBJ
Large and Medium Big Green Eggs , Black 18.5" WSM, Blue Weber Performer - Stainless, Green Weber OTG Kettle , Brinkmann SnP Pro, and a Stainless UDS. One retired Portable Kitchen grill.
Red Thermapen, Maverick ET-732, EdgePro Apex Sharpener.
Avatar is the original 1951 Weber Kettle