I use a charcoal fired Weber Kettle for small smokes. The chickens are normally 3.5-4.0 lbers each. At 225 -250 degrees, usually takes 2 3/4 - 3.0 hrs. I was using a probe thermometer, but now, its all time and they always come out around 165 -170 degrees after I "plug'em". Remember, they will still cook a bit after you take them out of the smoker.
Hope this helps ya out.