18 lbs is a huge chunk of meat. Using the "ballpark" time estimate of 1 1/2 hrs per lb. depending on the temp of your smoker, you're looking at 27 hrs. Cutting it in half would certainly reduce the time, but of course we're smoking to internal temp of the meat. In regards to "skin" which I believe you're talkin' fat cap, I'd remove it. The last shoulder I did I removed 1 lb 4ozs of fat. My reasoning was why cook what I'm not going to eat. You're just going to use a bunch of energy & extend the smoke time for something that's not going to be consumed. Then you also need to factor in the dreaded "plateau". You'll get a much more consistent bark formation since the whole surface of the meat will be the same all the way around. Not sure of your type of smoker, but I've been turning mine half way through just to even out the bark formation. This is just my method, and by all means not the ONLY method. It's all about experimentation.