To do those butts proper, even with smaller pieces, I believe you are still going to need a good 12 hours. The first time I tried small a pork butt, about 5 pounds, I learned you need that time. We had take out pork from the local BBQ joint. Besides, you really need all the fat on a big roast to melt away in the process.
Your time on a but is going to depend on your cooking temerature and how youlike it done. I would use a remote thermometer like one of the Polder digitals so that you can monitor you internal temp without opening the smoker. The more you open the smoker the more you add to the time.
If you want good sliced but 180 -185 is a good temp. If you want to pull it you need to bring the temp to at least 195 or 200.
You want to smoke them at an average of 230 to 250 degrees. The meat wil get to 160 relativly quickly. Then it will sit at that temp for a while as the fat renders down. Once it starts to move it wil do so quickly. I smoke mine for about 2 hours to let the rub set before I start to mop.
A fair estimate depending on your conditions is 4 - 6 hours.
Hope this helps a little.