I used a weed burner on mine. Took about an 1-1/2 hrs per drum. However, after doing it that way, as I was building my 55g UDS, I had to warm up the drum (it was to cold to paint) to be able to shoot some high temp primer to keep it from rusting out. So what I did was placed the drum upside down on some cinder blocks on their ends. I then put my deep fryer burner under the drum to heat it from the inside. It took less than a minute to get the drum up to about 120°F with the burner on the lowest possible setting where it wouldn't go out. Not saying it would work, but I would love to try it if I built another one, I would raise the flame to a higher setting and let it go longer. I bet it would heat up and get really hot. Not sure if it would get hotter than a weed burner or actual wood burnout though. Here is a pic of how the drum was setup with the fryer burner. I take no responsibility for your actions...lol.
I did it in the garage but if I were to try and really heat it up like I said, it would be outside in the open with nothing else around.
Another option is to just use regular firewood. We have a drum up at the VFW that we build a fire in every night and that sucker gets hot. We had it glowing red tonight down towards the bottom. Using regular firewood from out of the woods. Only recommendation there would be to drill your air intake holes first to allow good airflow. It will start to act like a charcoal chimney.
The fire wood or weed burner would probably be the safer way...lol. Just some ideas.