I think they should target 5 years, and in fact should lay out the product plan for those 5 years, building in scope for another 2-3 years more stuff if it proves wildly popular.
The way I would do it would be to first publish the core of the game, then publish an "Expanded Core" bringing back anything that didn't make the cut (including, notably, controversial elements like Dragonborn). That, plus support of Modules, Forgotten Realms support, and adventures, should give enough material for a couple of years.
Then do the Splatbooks, dividing them thematically, and expanding the classes in both the Core and the Expanded Core. By deferring these a few years, they give themselves the opportunity to give the line a shot in the arm as it starts to flag.
Again, the Splatbooks, additional Monster Manuals, support of a second setting, and more adventures should between them provide enough products for another couple of years.
And once you move beyond that, it's not unreasonable to start doing more experimental material - the 5e equivalent of "Book of 9 Swords", or whatever. But at that point, they're likely to be looking towards 6e anyway.