If the butt is cooked till done (and not pulled shy of done in order for residual heat to finish it during the rest) then one need not rest longer than an hour. Redistribution of juices will have occurred by then. That said, if more convenient time- or schedule-wise to rest longer, feel free.
If, on the other hand, one is pressed for time, then the rest period can be shortened fairly successfully by placing the butt on a sheetpan on the counter and simply tenting with foil--not wrapping it tightly. 30-45 min later the juices should have sufficiently redistributed. I would not recommend less than a 30-minute rest.