yeah, I agree with Clay, you could probably let spares go almost 8 hours. but you'd need to be sure it stayed around 225 - not higher. no way you could let baby backs go that long.
I personally prefer my ribs cooked at 275. Mine tend to get a little more dry at 225.
If you wanted to set it and head out for 8 hours, you need to be sure you know your smoker well. I often leave my smoker unattended, but usually don't do it for more than 3-4 hours max (unless I'm sleeping). If someone could be there to watch, you'd be much, much better off.
