Roasting four pork shoulders at the same time.

Pork shoulder at that oven temperature takes about 20-25 minutes per pound, depending on if it is bone in or boneless. I assume you want this to be falling apart, sheddable tender- so I would go the longer time, but start checking it after 45 minutes. It's pretty hard to over cook this particular cut of meat, so a few minutes extra won't hurt anything. You can also use a thermometer to gauge doneness. Officially it needs to reach a minimum of 145F internally. I find that this cut shreds best if it goes to at least 180F. If you want slices, choose the lower temperature. If you want shreds that fall apart, go higher, with the low and slow roasting temperature you have now. You can use a fork to gauge the doneness as well. When it is done, a bone-in piece will be falling off the bone. A boneless piece will practically disintegrate when the fork goes in. You should not feel any resistance at all.