Just did some preliminary testing. Vanilla world, no mods.
I set up four braziers, each with one item in them. I used a 2x4, long stick, heavy stick, and log. Unfortunately I forgot to actually time the exact length for which they burned. All I can offer you is what burned longest.
They were lit in this order - Log, 2x4, heavy stick, long stick, all within a turn or four.
The heavy stick burned out first. Then, about 40 minutes later, the 2x4 burned out. Then, about 90 minutes after the 2x4, the long stick burned out. And then, of course , the log burned longer than my patience allowed for.
So, that’s the best I can give you after only 5 minutes of testing or so. They actually burned out in the order of their volume, smallest first, largest last, and that would be my guess for what determines the length of time it takes to burn. Splintered wood, for instance, is a small volume and burns out very rapidly.
EDIT: As for breaking down items being better than burning the whole item. For Long sticks, at least, it seems that 2 heavy sticks (what you get from butchering a long stick) burn slightly longer (marginal, really, only a minute or three) than the long stick.