Times vary, the one last night at 250 took just a little over 14 hours. Sometimes they are ready a few hours earlier, sometimes later. Just wait for them to get tender, where the probe slides in like a hot knife through butter.
As for timing, I plan for it to be done 6 hours early, and wrap and put in an empty cooler to maintain temp until it is time to eat. That way it is very hard to be late.