Demolitions happen on the hour, every hour and they're ongoing. It's 45 days since the owner went in the house, but maintenance often adds time to it depending on length of the maintenance and depending on how long the server was "off" during the maintenance (we don't know). Even the owner doesn't get the exact time, they get emailed the earliest time it's going to get demolished.

So all you can do is keep checking I'm afraid! (teleport to each city on the hour when you can and scroll through the boards looking for a house)

Edit: Anyone that relinquishes a house would free it up straight away so it's still possible (although unlikely...) to get a house between hours if someone did that.