It will probably depend on how soon a number of obstacles are resolved, as well as timing of the planet orbits.
First, there's the question of the vehicle - with current propulsion options, it would be a multi-year journey there and back, so the vehicle used would need to accommodate that duration of time.
Second, supplies - with being so far away, there will have to be a way to produce supplies (water, food...) for the journey. There has been a lot of research in this so they are probably closest with this solution, but there are still long term and variety issues being worked on.
Third, bone degeneration - it is known that humans in space for long term missions experience bone loss (specifically the calcium in their bones starts to disintegrate/deteriorate), basically osteoperosis. This is one of the biggest obstacles they are looking for a solution for any long term space mission. There has been some progress in learning that muscle resistance exercise (with tension bands) helps to slow the degeneration, but it doesn't stop it entirely. If this isn't solved, there would be no way a crew would survive to Mars and back without having their bones become so brittle they wouldn't survive the pressures of re-entry (or make it back to Earth orbit without breaking limbs by bumping into things on the ship.
Finally - timing - the trip would need to be planned so the trajectory of the mission path and the Mars orbit occur when Mars is at a perigee orbit with Earth - this would ensure the fastest trip there and back.
Although these are a few of the challenges involved with taking a trip to Mars - when the solutions are found, the solution applications to benefit life on Earth would be enormous (especially the solution to osteoperosis - that could aid several people on Earth suffering from the condition if scientists figure out the key to this one alone).
Based on this - how long it will take will depend on funding, and the number of bright individuals that decide to put their mind to helping to solve these and other challenges. The sooner these are accomplished, the faster we'll get there.