Shouldn't take too long to get to 160 with that size butt. So much depends on how hot you let it cook--butts do much better at a low, slow temp. I usually count on 1.5 hrs/lb at about 225-240, but that's doing much larger pieces. Use a probe and/or a GOOD instant read thermometer to really know. Even with the probe (I have the Maverick 732--the best, IMHO) you need to be sure in different places with the instant read. Then, again, let it SLOWLY get up to 200-205 IF you're going to pull it. Takes the long, slow method to break down the collagen and fat to melt. IF you don't do that, your meat will not be that tender and moist.