The oxygen content of the Martian atmosphere is 0.174%, and that is after taking into account the fact that the Martian atmosphere as a whole is less than 1% as dense as Earth's atmosphere. This oxygen is thought to be created by the sun's ultraviolet light striking ice and stripping the hydrogen atoms off of the oxygen; we do not know of any life on Mars, and Mars would not be livable to humans or any other animal. Mars may have life on it, but the probes that we have sent there have not yet found any.
How long it will take for humans to get to Mars (and if it ever happens) is more a question of politics rather than science. The voyage would take about eight months, but it would be very expensive and quite dangerous, so convincing a government to pay for it, and also risk astronauts' lives, is not easy.
As more experiments are performed and better spacecraft are designed, the trip to Mars will become less dangerous and less expensive, but as far as we know it will always be expensive and never truly safe.
We do not know if humans will ever be able to live on Mars, even if we could build cities with air-tight domes that would keep a breathable atmosphere in. The reason is because Martian gravity is about 1/3 that of Earth. We know that humans cannot live in space forever in zero gravity: people on the ISS (International Space Station) need to be brought back to Earth in a year or so or they will eventually die from the loss of muscle that happens in zero gravity. We don't know how much gravity humans need, and if Mars' 1/3 of Earth's gravity is enough.
It will be possible to build a space station in orbit around Mars that humans could live on, however, provided thatthe space station were rotating to produce artificial gravity (the ISS does not rotate and so lacks artificial gravity).
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