Housing State Approximate Duration
Like New: 1 hour
Slightly Worn: 1-2 days
Somewhat Worn: 1-2 days
Fairly Worn: 1-2 days
Greatly Worn: 1-2 days
In Danger of Collapsing: 12-24 hours
It should not be greatly worn more than 2 days.
Outside of account reactivation 24 hours before the IDOC phase (in which case the house would have refreshed and no longer be in decay), the only thing I know of that is not a bug that interferes with the house collapse are if vendors with active contracts are attached to the house. But it goes all the way through the cycle and then says "demolition pending" on the house in that case, and stays that way until the contracts expire, then the house falls.
-Skylark