For torch-heated irons, most folks use a plumber's torch, but you can just as easily use a gas cooktop or propane grill; anything with a hot, clean-burning flame. We don't recommend using a wood-burning fire, primarily because it's difficult to control the heat, and they tend to crud up your iron with soot (that is unless you’re going with a brass head).
For electric irons, simply plug it in. It'll get up to branding temperature in 15-20 minutes. Essentially zero reheat time between impressions--it's got more than enough power to do long production runs. At operating temperature, you should be able to make a good, dark impression in 2-4 seconds.