In a recent support document from Microsoft, they have announced the end of support for Windows Phone 8 and Windows Phone 7.8 for the second half of 2014.
This seems to indicate that they are committed to the 18 month life cycle for mobile products but there is no mention of what's coming next either, though there are rumors of a new Windows device for the 2013 holiday season.
Microsoft is being a bit more transparent with their life-cycles it appears, unlike Apple when you really don't know when the 'end is coming'. It's fairly typical of the software giant though, as in the past years they have stayed consistent with telling everyone when they aren't going to support updates, or further development of a product (Windows Server, Windows 7, etc.).