If it's anything like a sirloin tip roat which I think it is. I would plan on about 4-5 hours to be safe. You want you meat to be rare on the inside and medium on the outer parts. You just have to remember we smoke by temp and not time. I like mine rare so I take it to about 135° or so.
I would give it 5- 6 hours at least, if it is done early you can wrap in foil and towels, shove it in a cooler, that will keep it at temp for at least 2 hours. I have kept Butts hot for 5 hours in a cooler before. Start early just in case.