"New Horizons launched on January 19, 2006, and it'll reach Pluto
on July 14, 2015. Do a little math and you'll find that it has taken 9 years, 5 months and 25 days. The Voyager spacecraft did the distance between Earth and Pluto
in about 12.5 years, although, neither spacecraft actually flew past Pluto
That's 19 years round trip.
BUT, if you want to stop there, you have to factor a lot of time slowing down (as opposed to just flying by at top speed). Likewise speeding up and slowing down on return.
You are talking about a lifetime - and that is assuming humans could travel at the same speed.