It was found that the first devaluation timer after a plot is demolished counts down to 5-6pm server time (original showed 6pm, but my looking around showed 5pm so maybe daylight savings adjusts it?), after that it goes on a 6 hour countdown cycle. There's no secret pattern, they're completely separate and you'll only know how much time has passed if you're there to see the house reclaimed by the system/demoed manually.