(Although sometimes these projects are built overnight, between departure of night and morning trains [1])
The Hokuriku shinkansen started construction in 1989 and opened reaching Nagano by 1997, just eight years later and in time for the 1998 Nagano Olympics. Of course they've built more extensions to it since then, most recently to Fukui just last year.
The Tokaido Shinkansen was on the drawing board in the late 1930s, and opened in 1964, after having been interrupted by the war and shelved for many years. A project starting today and opening in 2050 would be about that speed of development, although with the then-novel technology all being proven already one would hope things could be done a bit faster now.