It's not often that a series can say that it's actually older than the network it airs on, but that's indeed the case for Supernatural, which survived the merger of The WB and UPN into The CW. While creator Eric Kripke originally came into Supernatural with a five-year plan, that mark has already been nearly tripled, as SPN prepares to enter into season 13. The main driving force behind SPN's epic-length run is definitely its fiercely loyal fanbase, which never fails to follow the series over to whatever new day and time The CW moves it to.
Despite how long in the tooth Supernatural is, the ratings never seem to drop too far, much to the delight of The CW. For example, Supernatural season 12 was the fifth-highest rated show on the network for the 2016-2017 season, ranking behind only the four DC-based superhero shows that form the Arrow-verse. While ratings expectations on The CW are obviously much lower than on say CBS, that still doesn't make SPN's consistency any less impressive.
Last year, SPN co-lead Jared Padalecki drew headlines when he mentioned that the idea of ending the series after 300 episodes was being considered. To hit the 300-episode mark, Supernatural would need to complete season 13, then end with a shortened 13-episode season 14. However, this was later walked back by other co-lead Jensen Ackles, who said that 300 episodes was just the next big milestone for the series to reach, and not necessarily an end point. For his part, series regular Misha Collins even joked about Supernatural not ending until it hit 666 episodes.
Trying to predict exactly when Supernatural will end is far from an easy task, thanks to the ongoing success noted above. Most long-running series see their audiences taper off as the years go on, but SPN hasn't really fallen victim to that, as the fans remain as rabid as ever. It's telling that Supernatural is one of the only shows out there that is able to support year-round fan conventions featuring only its own cast and crew. On the corporate end, CW execs have repeatedly said that they're open to continuing the series until Padalecki and Ackles want to call it quits.
With that in mind, Supernatural ending may very well come down to just that, the two stars deciding to hang up their monster-hunting gear and ride off into the sunset in Dean's trademark Impala. A season 14 renewal seems all but certain, thanks to wanting to hit the aforementioned 300 episode milestone. After that though, things become harder to foresee, as neither Padalecki or Ackles have expressed a desire to leave their characters behind anytime soon. It's entirely conceivable that Supernatural will one day be heading into season 20. Whether that's a good thing or not is up to the individual viewer.