If you cook at that low a temperature, you should be looking at 12 to 14 hours as a guideline. 16 houes for a stubborn butt, possible. Also, the lower you cook the lower the done temperature usually is. However, this is not a harbinger of quality. Most of us cook a little faster, me, I am in the 280F range normally. I consider 225F the minimum, and that is too slow for me.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."