How long does it take to get to Mars?

How long would a manned mission take to travel to Mars? And what is the biggest reason for the mission not taking place in the near future. We sent men to the moon 40 years ago so obviously the technology is there to travel to deep space. Is it that they don't know how to build a ship that could carry enough fuel, food, water, etc.?

That page lets you calculate the simple Hohmann transfer orbit (the cheapest in terms of fuel and/or energy).

Pick the Sun as centre. Put the "altitude" of the initial orbit as 150000000 (150 million km) which is the distance from Sun to Earth.
Put 228500000 (228.5 million km) for the altitude of the final orbit.
Click on calculate. You will get something like 216 days.

This will calculate the parameters of an orbit that has its perihelion (closest to the Sun) at Earth's orbit and its aphelion (furthest from the Sun) at the orbit of Mars.

By symmetry, the Hohmann transfer orbit from Mars to Earth will take 216 days as well.

In both cases, the craft must leave one planet's orbit 216 days before the other planet reaches the target point of the transfer orbit. This forces the craft to stay on Mars for a number of days which can be calculated (depending where Earth and Mars are on their orbits)

Total is around 2 years, with a stay of many months at Mars.

There are two problems.
1) This is NOT a free-return orbit. If you miss Mars, the orbit takes you back to Earth's orbit, but Earth itself will not be anywhere close at the time you return.

2) Once on Mars, you MUST wait for the proper time (roughly 9 months) before starting the return trip. You cannot decide to leave earlier if things go wrong.

Other transfer orbits are possible, but they require a lot more fuel and/or different technology (initial chemical boost plus ion rocket, or solar sail, or...).

"Is it that they don't know how to build a ship that could carry enough fuel, food, water, etc.?"

They KNOW how to build it. They just can't afford to yet. And even when they do, it will still have to be tested (something always goes wrong with new designs) before being sent on a two year mission.
Something like the Apollo capsules would not do. For example, crossing the Van Allen radiation belts for a few days is not the same as being exposed to solar flares, solar wind and coronal mass ejections for two years.

Spending less than a week on the Moon is not the same as spending months on Mars (the lunar suits were damaged almost to their working limit by the time the astronauts came back).

It is going to be very, very expensive. And I mean "very".
Source(s):
There is a nice explanation here, along with a "map" of the Hohmann transfer orbit.