I would figure roughly 1.5 hrs per pound, so 16.5 hrs. plus or minus. plan to have it ready at least 2 hrs. ahead of time, then wrap it and let it rest. All of this is assuming that you are cooking between 225, and 250.
2 OTG One red one black
22 1/2" black Bar-B-Kettle with rotisserie ring
Brinkman dual zone charcoal grill
Webber Silver Genises B
3 burner camp stove