There's a rule in meat cookery that the tougher a cut is, the longer it takes to cook. That's not a major issue with pork, because most pork cuts are reasonably tender no matter how they're cooked, but pork shoulders are a bit more challenging than most. Pork shoulder roast cooking time really comes down to how you like your pork and how you're cooking it.
Choose Your Target
With any pork roast, you might potentially aim for three levels of doneness. Modern pork is safe when it's cooked to the same temperature as any other whole cut of meat, so you can roast it to the USDA's recommended 145 degrees Fahrenheit if you choose. That leaves the roast juicy and delicately pink on the inside, and you'll be able to slice it easily.
If you struggle with the idea of eating pork that's still pink, you can, instead, cook it well-done and have it finish at a temperature of 165F to 180F. It won't be quite as moist, but you'll be able to slice it quite thinly.
A third option is to go straight past well-done into pot roast territory. At this stage, with an end temperature of 200F or higher, the pork will fall apart easily into tender, luscious strands. It won't slice well, even after it's cooled, but it's tasty and tender enough that this usually won't matter.
A Rough Guide to Cooking Time
For a pork shoulder roast, cooking time is a function of the roast's weight, the end doneness you're looking for, and the cooking temperature. For example, the USDA's pork cooking chart suggests allowing 45 minutes per pound for a 3- to 6-pound shoulder roast at 350F, which would give a total cooking time of 2 1/4 to 4 1/2 hours to reach the recommended internal temperature of 145F.
If you're aiming for well-done roast, or if you're cooking one "low and slow" to use as pulled pork, or if you're using a full pork shoulder rather than the smaller pork butt roast, you'll need to allow more time. A 6-pound butt might take 8 to 10 hours on a smoker, for example. Most pork shoulder recipes specify a cooking time, but allowing an hour per pound for well-done or up to 1 1/2 hours per pound for pulled pork is a reasonably good estimate. To know for sure, use a meat thermometer.
Anatomy of the Pork Shoulder
You'll find pork shoulder roasts under a number of different names. The full shoulder might simply be called a pork shoulder roast or a "fresh picnic" roast, but those are inconveniently large for most families, and you won't often see them offered in the supermarket's meat case.
More often, they'll be cut in half into shank and butt roasts, which are the leg and shoulder ends, respectively. The butt portion is often called a Boston butt. Both halves are available boneless or bone-in, and they can be cut down into even smaller roasts for retail purposes.
The shoulder's a relatively tough cut of pork, partly because it's made up of well-used muscles and partly because it has a relatively large quantity of gristle and connective tissue. The muscles also run in many different directions, which makes it harder to get a clean, tender slice across the grain. Taken together, those characteristics mean that shoulder roasts aren't usually the best bet for diners who like their pork pink. At that level of doneness, the connective tissues don't break down, and the pork can be chewy.
Many pork butt recipes call for the cut to be cooked to the pot roast stage, for good reason. At temperatures above 180F to 185F, the connective tissue holding all those muscles together begins to melt down into natural gelatin. By the time the shoulder reaches 200F to 205F, that process is nearly complete. That's why pulled pork can seem moister and more tender than a well-done roast, despite being cooked longer and to a higher end temperature.
Slow-Roasting Pork Butt in the Oven
There are two useful methods for cooking a pork butt roast, or any shoulder roast, in the oven. The obvious first choice is to roast it. The conventional method, as spelled out by the USDA, calls for cooking the pork at 350F until it reaches the doneness you want. That method works, though you'll lose a lot of moisture as the pork cooks, and the surface areas may be dry and chewy as a result.
Slow-roasting is typically a better option, especially if you're aiming for a well-done roast or a pulled pork texture. Set your oven to a temperature of 250F to 300F, and cook it slowly until it reaches your desired level of doneness. Expect this to take anywhere between 1 and 1 1/2 hours per pound if you're aiming for a lush pot roast or pulled pork texture. You can either sear the pork roast on all sides to brown it before you start, or brown it in a hot oven at 450F to 500F for the last 10 minutes or so of cooking. Either option works.
Braising Pork Butt in the Oven
A lot of pork shoulder recipes call for the meat to be browned first and then cooked in a sauce or liquid of some sort. This is a technique called braising, and it's another way to arrive at a tasty, tender end result. The gentle heat of your oven is an ideal place for this to happen, as long as you've got a roaster or Dutch oven that's big enough to hold both the roast and the cooking liquid.
Cook the pork at any temperature or 250F to 350F, which will keep your cooking liquid at a gentle simmer. Allow about an hour of cooking time per pound, turning the pork at least once so the top and bottom cook evenly. The pork is done when you can stick a fork in it and easily twist away a tender mouthful.
Pork in Countertop Cookers
You can adapt your favorite pork butt recipes to a roaster oven, toaster oven or slow cooker with minimal tweaking. In a large toaster oven or a countertop roaster, use exactly the same times and temperatures as you would in a conventional oven. The only difference is that the heating elements in a toaster oven are physically closer to the pork, so you may need to cover it with foil to keep the surface from charring. In a slow cooker, a 3- to 6-pound butt should cook in 3 to 4 hours on high, or 6 to 8 hours on low. Turn it once midway through your expected cooking time; then test with a fork as the end of the cooking time gets closer.
Making Real Barbecue
If you have a grill or smoker, you have the option of doing true, old-school pulled pork rather than any of the "quick and easy" versions that crowd the internet's cooking sites and food blogs. If it wasn't brined at the packaging plant, you might want to brine your pork butt first. Otherwise, just cover the meat liberally with your favorite dry rub and let it rest in the fridge overnight, so the flavors can penetrate into the meat just a little bit.
Set up your grill or smoker according to the manufacturer's instructions and try to stabilize its temperature at 225F to 250F. Place it the roast in the grill or smoker, and close the lid. Cook at this low temperature until it reaches an internal temperature of 200F to 205F, which can easily take 8 to 10 hours. Let the finished pork rest in a covered pan or under aluminum foil for 15 to 20 minutes; then pull it and remove the bone and any large pieces of fat. Dress the shreds of pork with your favorite sauce before serving.