Pork shoulder low slow cook time
If you cook it too long uncovered it will lose moisture and dry out. If it is covered and you have a bit of liquid in there you could cook it longer and it would stay moist. You could turn it down to 200 too to slow the cooking process. 

EDIT:
If I was going to cook a piece of meat that long I'd opt for a braise instead of a roast as if you cook any meat dry too long it will dry out even if it's well covered. For pork some thin stock and white wine would do, or maybe cola or ginger ale which works well for ham.