re: A very long time
NASA has know a fire (Apollo 1), an explosion in flight (Apollo 13), an explosion at launch (Challenger) and a destruction in re-entry (Columbia) and it therefore must be very careful organizing flights.
If it's a new kind of mission (another planet), it's going to take a much longer time, without any possibility to return back early.
But it's true NASA wasn't interested in studying very long duration flights aboard the ISS, with recycled technology; even Russians are more advanced in that aspect.
Raoul