It would depend on the speed, of course. The New Horizons spacecraft was launched on January 19, 2006, and is currently scheduled to go into orbit around Pluto in July, 2015. That makes it about 9.5 years based on our current technology. (Rocketry has progressed hardly at all in the last 6 years.)
If you had a spacecraft which could accelerate at one gravity for half the trip and then slow down at 1g for the other half of the trip, you could arrive at Pluto in about 6 weeks. But there is no spacecraft design that we could currently make that could do this.