I don't think that Zelda U development time need to be pattern for future Zelda development time, because this gen is very strange for Zelda series. First Zelda U is first HD Zelda game, then it will be biggest and true open world Zelda, also Nintendo managed so many Zelda projects this gen while they working on Zelda U, OoT 3DS, WW HD, ALBW, MM 3DS, Tri Force, TP HD...you can say that some ports were made by different companies but Nintendo in some way certainly was involved in development, and all that certanly affected Zelda U devolpment. Also there are good chances that Zelda U port for NX affected development and launch of Zelda U.
With NX there will no be need for completely different Zelda games separate for handheld and home console, they can develop or port one game in same time for both devices, also Nintendo is now very familiar with HD development. Saying all that I can see new Zelda NX in 2019/2020.
But if Nintendo decide to pull again what they done with MM after OoT, and reuse Zelda U engine, assets and models for new Zelda NX we could have new Zelda game in 2018/2019. And I think there are good chances for something like that to happen.







