How long does it take? It takes the Earth one year to orbit the Sun and it takes Mars about 1.9 years ( say 2 years for easy calculation ) to orbit the Sun. The elliptical orbit which carries you from Earth to Mars is longer than Earth's orbit, but shorter than Mars' orbit. Accordingly, we can estimate the time it would take to complete this orbit by averaging the lengths of Earth's orbit and Mars' orbit. Therefore, it would take about one and a half years to complete the elliptical orbit above ( solid and dashed parts! ). Since it would be nice to spend some time at Mars, we are only interested in the one way trip ( solid line ) which is half of the orbit, and would take half the time of the full orbit, or about nine months. So it takes nine months to get to Mars. It is possible to get to Mars in less time, but this would require you to burn your rocket engines longer, using more fuel. With current rocket technology, this isn't really feasible.
In the nine months it takes to get to Mars, Mars moves a considerable distance around in its orbit, about 3/8 of the way around the Sun. You have to plan ahead to make sure that by the time you reach the distance of Mar's orbit, that Mars is where you need it to be! Practically, this means that you can only begin your trip when Earth and Mars are properly lined up. This only happens every 26 months. That is there is only one launch window every 26 months.
After spending 9 months on the way to Mars, you will probably want to spend some time there. In fact, you MUST spend some time at Mars! If you were to continue on your orbit around the Sun, then when you got back to where you started, Earth would no longer be where you left it!
In order to get out of your elliptical orbit around the Sun, and into Mars orbit, you will again need to burn some fuel. If you want to explore the surface of Mars, you will also need fuel to get your lander off the surface of Mars. On the first trip to Mars, it is necessary to bring all of this fuel with you to Mars. ( Maybe someday we could manufacture rocket fuel on Mars ). In fact, you can only land a small part of the ship on Mars, because landing everything on the surface and lifting it off again would require enormous amounts of fuel. Therefore, you will probably leave part of the ship, including all the supplies for the trip home, orbiting Mars, while part of the crew goes to explore the surface.
Just like you have to wait for Earth and Mars to be in the proper postion before you head to Mars, you also have to make sure that they are in the proper position before you head home. That means you will have to spend 3-4 months at Mars before you can begin your return trip. All in all, your trip to Mars would take about 21 months: 9 months to get there, 3 months there, and 9 months to get back. With our current rocket technology, there is no way around this. The long duration of trip has several implications.
First, you have to bring enough food, water, clothes, and medical supplies for the crew in addition to all the scientific instruments you will want to take. You also have to bring all that fuel! In addition, if you are in space for nine months, you will need a lot of shielding to protect you from the radiation of the Sun. Water, and cement make good shielding but they are very heavy. All together, it is estimated that for a crew of six, you would need to 3 million pounds of supplies! The Shuttle can lift about 50,000 pounds into space, so it would take 60 shuttle launches to get all your supplies into space. In the history of the Shuttle, there have only been about 90 launches, and there are less than ten launches per year… So with the shuttle, it would take six years just to get the supplies into space. For this reason, you would probably need to develop a launch system that could lift more than 50,000 pounds into space. Even with a better launch vehicle, it is unlikely that you could launch the Mars mission all at once. You will have to launch it in several pieces and assemble them in orbit.
Second, you are going to be in space for an extended period of time, and there a physiological consequences of being weightless for long periods of time. For one, your muscles do not need to work as hard. In response to being used less, your muscles begin to shrink or atrophy. Remember, your heart is also a muscle, and pumping blood around your body is easier in the weightless environment of space, so your heart gets weaker as well. On an extended space voyage, your muscles might become so weak that it would be difficult for you to stand upright once you return to an environment where you are subject to gravity.
Just like your muscles have to do less work to move you around in space, your bones are not needed as much. The main function of your skeleton is to support the weight of your body. When you are weightless in space, your body realizes that the bones are not being used as much and they begin to lose calcium, and become more brittle. These are serious effects which may impair the ability of the astronauts to carry out experiments and tasks when they get to Mars, where they will be subjected to gravity again.
In order to study these physiological effects of long duration weightlessness, you need to do experiments on people who have been weightless for extended periods of time. Currently the Russian Mir space station is one place where astronauts can stay for extended periods of time, and research into these effect is ongoing. But since you will need to conduct many more experiments, and you will also need a place to assemble the mission, it will probably be necessary to construct a larger space station to be used as a staging ground for the mission to Mars.