Fun fact: Wood burns insanely fast in D&D. Torches burn out in either 2 minutes or 2 rounds, depending on how you interpret their thickness, and you can burn down a 10ft thick section of wooden wall in under 20 minutes. Ponder that for a second. A ten foot cube of wood takes 20 minutes to burn to a crisp. A 2-ft diameter log burns to a crisp in 4 minutes. Why? The 1d6 damage dealt by being on fire is doubled and ignores hardness; 7 damage per round adds up quickly.

There is a flip side to this, however. The damage you take from catching on fire isn't explicitly fire damage, meaning that the block of wood would take 1 point of damage every 6 rounds or so, making for a reasonable burn time. On the other hand, if I'm not misinterpreting this, you could set a Fire elemental on fire, and it would burn to death if it failed its saves.

Finally, an interesting bit of stupidity: You can not only set a rock on fire and have it burn indefinitely, but you can set literally any unattended object on fire (since they always fail their saving throws, being unattended) regardless of its inflammability or lack thereof. This may or may not be intentional: Since holding a torch near anything doesn't technically cause them to catch on fire, the ease of adventurers setting the world on fire accidentally is minimal. Fire Elementals, on the other hand, set any inanimate object they touch on fire that may or may not ever burn out.