OS X is a brand. It's like saying "when will they change the name of the iPhone?"
OS X was a play on words (it's pronounced Oh Ess Ten) because Apple was on version 9.x of the Mac operating system.
When Steve Jobs returned from NeXt (notice the capital X) he brought the Unix-based operating system that they had been developing.
OS X wasn't called OS 10 because it was a completely new kernel based on Unix. So the play on works was to call it Oh Ess Ten but write is as OS X to signify that it was the operating system following 9.x but was Unix based.
So, unless there is a brand new kernel that gets introduced, the name will never change from OS X. We are currently on OS X 10.9.2. You would pronounce this Oh Ess Ten, Ten point Nine point Two. The next major release will be either OS X 10.10, or it could be OS X 11.0 (unlikely). There will be no OS 11.