Thickness. I have triple wall.

It really depends on your stove and stovepipe and how you run it. Burning really dry wood helps, and I imagine hardwoods would help. But other factors include how tall your stovepipe is, how much of that stovepipe is insulated, how hot you run your stove, whether you do a flash fire every day to burn out any buildup, etc. all these things factor in. Our wood is dry, stovepipe is short, most of it is insulated, and we run it fairly hot most of the time and do a flash fire almost every day. So I still haven’t had to brush it out. But it’s important to at least check it every year and when I check ours has been good. Others who burn similar wood but have a longer stovepipe or crank their stove down more than we do have to brush theirs out once or twice a year. Your mileage will vary.