Nasa will launch a mission to Mars in 2020. This may sound shocking until you learn that this will be another unmanned mission sending a robotic rover to explore the surface and perform experiments.
The next manned mission to another terrestrial object will probably be a 2024 mission to the moon. We should be good at this, and it is good practice for manned exploration of other terrestrial destinations.
The soonest time we might imagine a manned mission to Mars, even just for orbit, would be 2033 returning in 2035. The departure and return times would normally be about 26 months apart. Based on the orbit of the Earth and Mars, 26 months pass between minimums when travel time and energy required would be ideal. Travelling at the wrong time could increase the time and energy required by double. Ideal travel windows for the shortest possible mission (26 months) occur twice a century.
Consider that picking the ideal time would allow the mission to leave Earth and travel to Mars and return in just over 26 months. Picking the worst time for the mission could require in excess of five to ten years to complete.
A variable of great import in a trip to Mars is the type and quantity of fuel to use. To achieve the fastest travel time, you would want to accelerate until half way to Mars, and then decelerate until you land. You would need of course, to have plenty of fuel left to get home.