Brian...4-5hrs in a dry cooler. Wrap the butts in Heavy Duty Foil in a few layers then put towels on top...I heat my towels in the dryer for 20min or so just for extra heat ad hold time.
Also...take the butts to 205 or so...you can minimize the hold time the longer you keep it on the smoker. Do you have the butt foiled on the WSM right now??