This is why people like Scott Kelly should be considered heroes. They are pushing our knowledge about manned spaceflight, and doing to at considerable personal risk and effort. But as a result, we are getting more knowledgeable about the problems, and hopefully solution approaches. That stays on the ISS of 6 months or even a year are somewhat a routine is quite an achievement. And we might find out, how to do even longer stays in zero gravity. With these results we then can set mission limits for a potential Mars mission. Of course one obvious solution would be to fly faster. Musk hinted at his BFR presentations of a 3 month flight time which would be less than the routine stays on the ISS.