Yesterday I put a 7# Boston Butt on to smoke. I started it at 11AM and according to the the "rule" of 1.5 hrs per pound, it should have been done at 9PM. Well at 9PM the internal temp was 174 degrees. Thank God for my Maverick thermometer. If I had relied on time alone, the pork would have been seriously undercooked and really tough. At midnight it finally got to 190 and it was at that point I decided that I was too tired to go for the final 10 degrees. I pulled it out, rested for 30 minutes and pulled it. It was delicious, juicy and tender. Not as tender as I wanted it, but still very good.
Don't rely on time. Get a good thermometer.