4E was functionally out if print by 2011. Still combine that with the start of the 4E phb about the next decade it seems the assumption is 10 years for a D&D edition.
Spitting out 3 editions in 7 or 8 years doesn't work as they did 3 editions of Star Wars as well.
If you just looked at prior history in 2000, 10 years seems about right. The core of AD&D 1e came out over 3 years, but regardless of which year you pick (77-79) there was a roughly 10 year span until 2e was released in 1989. Then 3e was released in 2000 (though IMO that timing had as much to do with TSR's corporate struggles as it did for a "natural" span between editions). Looking at it from 2000, expecting a new release to drop roughly 10-11 years later because that was how long 1e and 2e both lasted seems reasonable.
One thing about the 3.x run for D&D is that it set up some bad expectations about publication schedules. 3.0 was barely 3 years old when 3.5 was released. 3.5 lasted 5 more years before 4e was released. 4e then got roughly another 5 years (6 if you just look at when 5e was released compared to when 4e was released, but for an entire year before the actual release it was all "D&D Next" hype, so functionally only 5). And since Wizards had internalized that the financial problems that TSR faced was partly due to diluting sales across setting lines, and that their surveys said that people wanted "rules crunch" in the accessories they bought, they were dropping a book a month full of rules changes onto the games. 3e couldn't handle the weight, which is why it got 2 revisions over the span of 10 years (just 3 years later when 3.5 dropped, and then one in 2008 when they reworked the engine to be more robust to the grind of dropping monthly rules updates and called it 4e - despite everything I enjoy about 4e it's pretty clear that one of the intents behind 4e was to make it easier to add features to the game quickly. I'm pretty sure I even remember someone - maybe Dave Noonan or Mike Mearls on their podcast back when he did that? - explicitly saying that. One of the compromises for 5e is that they moved back to an engine that is harder to slot new rules into, and changed their publishing expectations instead).
I would not be surprised at all to find out that internal plans for 5e originally expected to have to release a "5e revised" this year (5 years in) to tweak the sales numbers. But the sales game they're playing has definitely changed for a number of reasons both involving how Wizards has been handling D&D internally and how the game has seen renewed popularity independent of things that Wizards has done on purpose. (2e did get a "revised" edition around 1995 with new layout, art and some errata incorporated into it - if it weren't for the fact that 5 years in the PHB is still selling incredible numbers I could have seen them doing that this year. But from a publication perspective you don't want to do that when it's still selling so well - you want to keep that bullet in your chamber for when you need the sales).
