IMHO, metal houses are nice to own, but offer minimal protection over wood.
If I remember correctly (and the numbers haven't changed recently), wooden houses start decaying 24 hours after the last player activity inside them (opening doors/chests, lighting camp fires). After 24h of inactivity, they will start decaying from top to bottom, 3h(?) per layer. Metal won't start decaying until 48h have passed, and once it has started decaying, each layer(story) will last 12h.
So with a wooden structure, you better log in daily, or you might be missing your penthouse. :)
Edit: Just found Helk's post. It's from November, so I'm not sure if it's still accurate:
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