The Sumatran is fairly quiet Ive found. At 12 minutes, you really havent baked anything. Its usually upwards of 20 minutes that you start to get into baking territory.
Popper roasting is very sensitive to ambient temperatures. When its cold, try to speed up the roast with a larger mass of beans. When its warmer, a lesser mass of beans is the go.
Most will stop the roast when they hear the first few snaps of 2nd crack. Depending on the bean in question, I will go into rolling 2nd crack. If you let 2nd crack go on too long, you end up in the Spanish roast territory, a.k.a. burnt. Sometimes you really want the origin characteristics to dominate, so you stop it when 1st crack finishes. Always remember that the popper will often turbo roast the beans. If youre getting 6-7 minute roasts with the max capacity, youre doing fine. Try to experiment to find the max capacity of your popper. If the thermal fuse cuts off mid-roast, then you have too much bean mass. Drop it back 5-10 grams and try again.