Planet-to-planet distances are a major variable, since both orbit the same star. It depends heavily on the launch window; outside of launch windows, you'd need Star Wars-grade rocketry to perform a transfer. And even with windows the flight time is usually 3-5 years. Modern missions may include multiple fly-bys of multiple planets to perform "gravitational slingshot" maneuvers - Cassini, for instance, circled Venus twice, then came back to Earth, and only then built enough momentum to be thrown towards Jupiter and Saturn.