From my experience, such world change happens, in average, twice a month. I haven't ever seen it happening twice in less than 7 days, and sometimes it took about three weeks for the jars to turn back to their breakable stage. But I've noted this WC down 20 times in 10 months, so the average interval could be seems to be about 2 weeks. Please, note that this interval can be shorter or longer based on how many days long does it take to kil for the game world will to kill the necessary amount of honour guards.