Hundred-millionaire Dennis Tito - the first commercial space flyer, who flew to the International Space Station on a Russian rocket in 2001 for $20 million - has announced through his new organization the "Inspiration Mars Foundation" (which doesn't appear to have its own website yet) that he intends to launch a 501-day roundtrip manned mission to Mars in 2018, and will be giving a press conference about his plans next week on the 27th. The preliminary announcement does not make clear whether this will be a landing mission - which would radically increase the cost, complexity, and danger of the endeavor - or a purely orbital one, which would be far more likely to succeed in the given timeframe. However, despite Mr. Tito's resources, there are many technical and economic reasons to be skeptical about the likelihood of a five-year schedule for such an undertaking.
First, we can divide the breakdown into the two cases - landing vs. orbital:
In order to land on Mars, you need to deliver a habitat and all necessary resources to the surface ahead of time, and then you need a manned lander/ascender to deliver and retrieve the crew. None of this infrastructure exists as of today. No functioning planetary landers capable of delivering the mass of a crew capsule and then launching back into orbit from the Martian surface exist right now, although there are plans to evolve the SpaceX Dragon capsule to fulfill such a role over an indeterminate timeframe. The likely expense of not only developing, but building and launching this infrastructure would run into the billions of dollars - at least an order of magnitude greater than Tito's personal fortune - and I doubt very much that it could be completed within five years.
Orbiting Mars is another story. All you need is a reasonably spacious, robust, long-term habitat capable of sustaining the crew for 501 days in such a way that they don't go crazy, the systems don't fail in any way requiring resupply from Earth, and the crew is protected from radiation. This doesn't exist either right now, but it's much less of a tall order than the landing infrastructure, and could theoretically be met or at least augmented by inflatable modules such as those offered by Bigelow Aerospace. Moreover, very powerful rockets will be needed to deliver the material, so we're talking either Delta IV Heavy or a Falcon Heavy (which is set to have its maiden launch this year), both of which are expensive (although Falcon would be considerably less so). Nonetheless, I would be highly skeptical that this endeavor could be funded at less than a billion dollars, which means Tito will need equally resourceful partners to make it happen. However, given a billion dollars or so, I think the five-year timeframe for orbiting Mars is doable.
On a scale of 1 to 10 of credibility, with 1 being a complete joke with zero chance of ever happening and 10 being a fait accompli, I would give the orbital scenario - based on the slim preliminary info I'm hearing - a credibility rating of 5. The landing scenario, however, only gets a 3. I will discuss it further next week when additional information is forthcoming.
6:19 PM PT: There is a middle-case between a full landing landing mission and a purely orbital one, I just realized: You could land on Mars very briefly just to put footprints and take pictures and then immediately return to orbit. This means you wouldn't need a surface habitat, but only a lander/ascender. Still, this would be expensive and almost as dangerous (since most of the danger is on ascent and descent), so it's only modestly more credible than the full landing scenario - 3.5 on a scale of 1 to 10.
6:25 PM PT: It's also not clear whether Tito himself will be on the mission. The fact that he would be 77-78 years old in 2018 is a real issue if he intends to do that.