|Shiny! No, seriously. Very difficult to photograh.|
In absolute terms, it's actually ahead of schedule, with "D&D Next" announced only four years into 4E's rocky tenure. On the other hand, as changes in the gaming landscape and commercial pressures have forced the rights-holders into ever more frequent re-thinks, you could say it's pretty much on the curve. Basic to AD&D was an early blip at a mere three years; AD&D 2E arrived twelve years after that; 3E eleven years later; 4E eight years after that; and now 5E lands a mere six years on from its predecessor, with two years of open play-testing built in to that. (That list doesn't include mid-cycle "refreshes" which I don't consider different or exciting enough to qualify as a new edition.)
From a gamer's point of view, these new editions have all brought different things to the table. AD&D 2E was very much a tidy-up of a ragged but beloved set of rules; 3E essentially rejuvenated the whole brand, which was dead in the water after years of mismanagement, bringing a ground-up re-design which combined modern thinking with all the classic D&D tropes; and 4E, the most recent incarnation before today, was then arguably a response to d20's/OGL's market saturation, introducing a radical new mechanical approach but in doing so sundering the community -- and its paying punters -- into multiple camps. Consequently, the 5E Player's Handbook lands for the first time in a market where D&D isn't necessarily the biggest name in town.