I would imagine that as long as your Cr-48 stays running, it will continue to get updates. If you are on the dev channel, you should have gotten an update yesterday evening or today. Now that I have said that, I decided to log into my Cr-48 and check to see how long it takes to see that there was an update released last night :)
I suppose there may come a time when the tweaks to Chrome OS start to exceed the capabilities of the hardware in the Cr-48, but so far, mine is still keeping up. As long as Google releases updates to Chrome OS, I imagine that the Cr-48 will install them. Nothing I have seen or heard would indicate otherwise.
I might have a couple of keys on my Cr-48 where the letters have been rubbed out, but the machine still runs and gets updates very shortly after my Acer AC700 gets them. Sometimes the Cr-48 gets them first. Your mileage may vary, but I do not think there will be a hardware cutoff date where the Cr-48 gets abandoned. I look at it as "almost 3 years, and it is as up to date as a brand new Chromebook". That says a lot about how Chrome OS was designed.