In general, Gravity Assists do not reduce the amount of time, unless you are going really far out there. For instance, Galileo took 6 years to make it to Jupiter after making 3 flybys, one of Venus, two of Earth. Comparatively, New Horizons made it there in just over a year, Voyager 18 months, and Pioneer about 2 years. However, there is considerable fuel savings from doing the flybys.
However, if you are already going to pass through the orbit of an object, then a flyby could speed you along. Thus, Voyagers are going faster through space than New Horizons will ever be, because of the extra flyby of Saturn the Voyagers received.
Getting to Mars would not have any candidates for speeding the journey time to it except perhaps the moon, as Mars is the easiest planet to reach from Earth. A flyby of Jupiter would probably make a mission to Saturn faster, if you line it up just right.