The average lifecycle of video game consoles and handhelds in the modern era seems to be around 7 years. Nintendo was one of the main players in establishing the pattern of release between consoles and handhelds, where consoles are released in the middle of a handheld's lifecycle and vice versa. I'd place the first announcements of the 3DS' successor to come around mid-to-late 2017 for a 2018 release.