In my story, the land of Koyokuni has a Japanese-style castle, Akimachi Castle. The castle - and indeed the entire city of Akimachi - is uninhabited, though the land is being repopulated after the end of the war in which it became completely depopulated.
The current ruler of Koyokuni is effectively a reincarnation of the ruler at the time Koyokuni was defeated, and as a result has bad memories of the castle in which the former lord and his wife were tortured to death. The current lord's girlfriend suggests that if the castle holds such bad memories for him, perhaps they should just burn it down and have done with it, and he agrees.
So, the "Akimachi Castle" which is burned is the main tower of a Japanese-style castle similar to Osaka Castle, built with the traditional materials. It is in good repair and partly furnished and equipped in the Japanese style, though it has no food reserves (it had been repaired and maintained over the past 17 years by automata, but not resupplied). The bailey of the castle is large enough that the main tower will be far enough from the buildings of Akimachi that the few people present can easily prevent them from burning.
Nobody is trying to prevent Akimachi Castle itself from burning.
My question is: How long would it likely take for the main tower of this castle to burn to the ground after a determined attempt at arson in which its ground-floor timbers and other flammable items are set alight?
References to accounts of similar historical events such as the fall of Osaka Castle in answers would be helpful, but I haven't been able to find any which say how long a Japanese castle might burn before it collapsed or burned out.