How long to travel to Mars from the point of view of those traveling?

T G asked:

I’ve heard that it will take roughly 9 months to travel to Mars. But, time travels faster when farther away from large gravitational fields. How long will it take from the point of view of an astronaut making the trip? What will be the maximum difference in the speed of time from an astronaut on the journey from the speed of time at mission control on earth?