How many hours would it take a Cessna 421 C to go 600 miles?

Depending on winds a little less than 3, we used to flight plan at 200 knots, true airpeed in the C model I flew.

I presume you mean statute miles and not nautical miles. For flight planning purposes (weather, atc routing, etc) plan on 2 1/2 hours, plus or minus the effects of wind from the time the plane leaves the runway until it lands. Add extra time for taxiing for takeoff and after landing. At busy airports, atc routing and ground time is usually longer than at rural airports. Average wind depends on both the area of the flight, the direction of flight and the time of the year. At flight altitudes the wind almost always blows. In average weather with average taxi times on the ground, the trip might take anywhere from 2 to 3 hours total. In other words, more information is required

TAS @ 75% is about 240 Kts. So it will take you about 2 and a half hours. The wind will change this speed.

A Cessna 421C crusing speed ranges from 275-300 MPH i would take about 2 hours give or take, deppending on the weight and winds.