The one I did last weekend was 6.51lbs and took 12 hours. So if you want them still warm from the initial cook tomorrow at 3pm, I'd start them tonight and cook them over night. I have held butts before in the oven for 4-5 hours. I set the oven to 170, wrap the butts really good in foil, then when I put them in the oven I turn the oven off. I'll check every hour or so and just gauge the temp by touch to determine if I need to turn the oven back on for 10-20 minutes. Putting them in a cooler wrapped in foil then wrapped in towels has the same basic result, but I think its easier to just use the oven. The butts will be hotter than the oven temp when you put them in, so the oven will hold a nice warm temperature for quite a while. Just as long as you keep them about the danger zone cutoff (140 degrees) you are in good shape.