Seven hours plus any server downtime that occurs after the sign changed to In danger of collapse.
If there is a rental vendor inside the rental vendor owner is allowed 9 days to come and collect their cash/items the rental vendor was holding, so the house will refresh rather than fall to allow for this.
Some idocs get bugged and go through all stages of decay only to refresh at the last minute. Bugged idocs have been known NOT to fall for years.
One good way to tell if an Idoc has a good chance to fall or not is to find out who owns it and then see if any other idocers are around.
If the sign says:
Owner: OSI
and:
There's no other Idocers to be seen in the last hour after it turned Idoc it is probably a bugged idoc that has little chance of actually falling. Professional Idocers don't waste much time sitting around waiting for these, that's why you won't see them wasting time there.