If you so much as open a box or a door, or a campfire or furnace is on, or there is any activity in a house at all it will not decay ( by you or anyone else )
Wood
-Decay 'grace period' 24 hours
-Each 'level' of the house (top to bottom) ~3 hours to decay
Metal
-Decay 'grace period' 48 hours
-Each 'level' of the house (top to bottom) ~12 hours to decay
This means a 3 story wood house from the moment there is no activity would take approx. 36 hours for it to completely disappear (including foundations) but as long as you did anything within a 24 hour period it wouldn't need repairs at all.
A metal house of the same dimensions would take approx. 96 hours ( 4 days ) to decay
Please suggest some alternate timings if you wish
I would like maybe 48 hours for walls and like 56 hours for foundations. That's up to you though. I do feel like the people who have class and jobs are almost discouraged to playing this game. I would like not to get on a single day and have my house still be there. I don't mind every other day joining in and starting a campfire and opening a couple doors, and replacing some metal doors that might be blown down. I hate joining in to play after playing for like 6 hours to get a house built just a day earlier, to be confronted with no home.