It depends on the size of the roast, usually 8-10 hours, if you crank the temp up some you can shorten it some, but I like the low and slow, IMHO it makes it more tender.
If I put one in first thing in the AM (like 0700) it is usually ready to foil around the 4-5 hour mark, done around 3-5 in the afternoon, FTC (Foil, towel,Cooler) until serving time.