My first smoke ever (which was on my homemade 20" diameter classic offset smoker) was a single pork butt and it took me ~ 14 hours (for my 7lb butt). Since then, I have only smoked 5-5.5 lb (max) butts, and it takes me 11-12 hours no matter how many I put in there. About a month ago I had 7 of these and it still only took about 10-11 hours or so. If you have individual probe thermometers for each butt, that's great. If not, you could try probing your largest butt (no "butt jokes" here) and just pull them all off once the largest reaches about 200F internal. If the smaller ones go a little longer, it's not going to hurt them. Remember to post some pics.