- 1974: 0E (White box edition)
- 1977: 1E (Monster Manual first published)
- 1989: 2E (Core Rulebooks first published)
- 2000: 3E (Core Rulebooks first published)
- 2003: 3.5E (Core Rulebooks first published)
- 2008: 4E (Core Rulebooks first published)
- 0E - 1E: 3 years
- 1E - 2E: 12 years
- 2E - 3E: 11 years
- 3E - 3.5E: 3 years
- 3.5E - 4E: 5 years
- (3E - 4E: 8 years)
- 4E - now: 3 years
The average interval between major releases is 6 years or so. A release of 5E in 2012 wouldn't be unheard-of fast*. A 2013 release would be more reasonable, and from a marketing standpoint possibly easier to sell, given that it would tie 3.5's record. That's where I'm placing my bet. Will we see it before Christmas 2014? I think that's a lock.
* Super-duper conspiracy theory: Monte Cook was working on 5E all the time, and was brought on board to polish the design after presenting it to the WotC brass nearly fully-formed, like Athena from the brow of Zeus. Seeing the writing on the wall re: the white elephant that is 4E, they jumped at the chance. Complete speculation on my part. Complete.