I have an electric and usually do 2 10-12 lb guys at the same time. I have found that the 1.5 hour / pound is at least a good starting point.
I like to start mine around 5:00 pm and give it good smoke for at least 4-6 hours, make sure it gets past 140 degrees for safety, and then turn it nice and low (180ish?) so that I can get a good night's sleep. (With my smoker and Maverick digital read outs beside the bed just in case). Then when I wake up, I can check the internal temps and pop it back up a bit, or leave it as desired. Mine are usually done around 9-10:00 am.
So start it nice and early because after wrapping, it will keep beautifully for hours. But yes, temperature is your only true guide.