You can always foil during the cook if the butt is taking too long.
A normal cook for me is around 12 hours (I start it out low and slow ...240 grate on the overnight, and ramp up the temps wide open in the early am).
But if I was starting at 6am and wanted to serve the butt by 6 pm, I'd go without water and shoot for 280-290, maybe even a bit higher if you can.
Finishing too early is never a problem, but finishing too late can be.