at 225-255 you will be looking at about 1.5-2hrs per pound
at 260-285 you will be looking at about 1-1.5hrs per pound.
at 290-320 you will be looking at about 45mins-1hr per pound.
Edit - That would be per pound of the pork butt (3 5lb butts will cook in about the same amount of time of 1 5lb butt, but 1 15lb butt will take longer to cook than 3 5lb butts)
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