Gping to give my ne 18.5 inch bullet a ride this weekend. On the kettle an 8 lb butt takes 9 hours. How long will it take in the bullet? Throwing three racks of ribs on as well.
Each one is different but I would start with 1 - 1 1/2 hours per pound. If it gets done early you can foil it and put it in a cooler while the ribs are cooking.
For a 8 lb butt to be done in 9 hrs on my 18.5" WSM I run between 300 - 325 measured @ the top vent.. I do a Minion start with all vents wide open. Close down the lower vents around 275-280 and let it climb till it settles down. Usually around the 6 or 7 hr mark the color looks good then I'll foil till probe tender. That only takes a few hrs at those temps.
Good luck on the ride and enjoy!
Edit: I should add I use an empty foiled water pan.
Tim