I'd go by internal meat temperature, if I were you. Smoking meat has such a wide range of variables that it's hard to go just by time. How hot your fire is, how hot the day is, how big the piece of meat is....
I did learn something new the other day, though. Internal meat temperature continues to rise for a while after you remove the meat from the heat. So if you take it out
exactly at the doneness you want, the internal temperature will actually continue rising for a little while afterwards. So don't wait until the perfect temperature to remove the meat from the heat.
Our pastrami was a pretty large chunk of meat, and so I took it out at 158 F. The temp continued to rise until 165 F.