I have seen differing numbers on this, but the two numbers that keep popping up are 260 days and 6 months. This is just for the one way trip at the time when Mars is closest to the Earth. Once you reach Mars, you need to wait another 1.6 years before the two planets are closest together in order to make the return trip. So, in total, 6 months to and 6 months from, then 18-20 months on Mars to wait until the planets are in the right position.
Various propulsion systems are being contemplated/designed that would bring you to the red planet much more quickly. Among them, are anti matter drives that could get you there in days or even hours. The problem with this out there technology is that it would require about 1 kilogram of anti matter to power this trip, and the Earth doesn’t even have one millionth of this amount. Another technology being considered is the Solar Sail. This provides an unlimited amount of flight time, but makes the trip way longer than traditional rocket fuel. Nuclear powered rockets are much more powerful and could get us there quickly, but political and environmental concerns interfere in exploring this option further. Lastly, an ion propulsion system, using charged xenon gas could get us there more quickly. I am not sure of the exact time, but I have seen estimates of 30 to 90 days for a one way, or round trip.