Hopefully it’s thawed already because it will take two days to thaw all the way through in the fridge. 13 pounds is huge in the slow cooker, and should take about 13-14 hours on low or about 11-12 hours on high. Might be better off braising at 275-325 F in the oven, see this recipe for the general gist www.theseasonedmom.com/braised-pork-shoulder/. Should only be 4-5 hours this way and remain tender and juicy.