I only cook flats, and just did a 4.5# last weekend.
WSM pegged at 225* or so, I believe it was on for 7 hours (wrapped at 165*, then to 190*) after that it was in the cooler for 2 hours.
I've had 6+#r's take 10 hours, and 7#'rs take 6 hours.
Every one has been different, and a second probe...