Indeed it was planned to be introduced within the month of the announcement. However with all the problems with the server change its been delayed a bit. Expect a Sim News post when the timer has been activated.
When this is introduced, will there be a feature to allow the cities owner to see how much longer they have before the city collapses? I could see this coming up down the line when blank cities are put up on the market. The buyer might purchase the city to only have it fall a couple days later before they can even get out onsite to build something.
Cities currently have a made on date (like all producable entities). As for the market there was some discussion about that. Possible that only cities with more than 1 month of time left will be allowed to go up on the market. Will be dealt with by the time its released.