This has been discussed numerous times here, and it all comes down to the fact that nobody knows. It's tough to compare OS X to earlier versions, since a point release of OS X is more like a full release of a pre-OS X version.
Steve has said that OS X is good for 10-20 years (he said it several years ago, so subtract a few from that number), so it's a matter of when they get tired of the OS X moniker or there's a significant leap forward in the design. When that will be, nobody really knows. I doubt the visionaries at Apple even have a firm idea.