That is a tricky question. The goal is to get the entire barrel as hot as possible. I might have gone over board on mine, but this is what I ended up doing;
I put in about 8 splits of firewood and lit her up. The billowing smoke only lasted till the fire got going, then it was pretty clear after that. I did get a burnt paint/chem smell as the exterior paint burned away and ashed white. I had the barrel on 3 6" tall blocks to keep it off the ground and the grass underneath was still burnt, I kept spraying water under the barrel just to be safe. I let the wood burn for about 4 hours, I did add a few more splits once it got going. The problem was that only about 3/4 of the lower part of the barrel got hot enough for the paint to completely ash over. I didn't notice this until the next day as I let it cool over night. This bothered me as I wanted to be certain that the entire barrel was heated enough, so out came the weed burner and I torched it with that. The rest of the paint ashed in a hurry, and I was able to get the metal red hot in short order.
Next time I will just wash, torch with weed burner, and wash again. And by the way, my barrels are marked as being food grade holding "biodegradable, Kosher Coconut Oil." I got the barrels from a steel recycling plant. Since I didn't get them directly from the original user/owner there was no way to be certain nothing else was in them so I may have just gone overboard.