Valery Polyakov's 438 consecutive day single-flight duration record is still only about 14 months, a bit shy of one-way to Mars. But, that probably gives a good look at the effects of flights about that long. Still, a mission to Mars is two of those, plus some time on the Martian surface in low G and high rad.
Maybe the first test mission for the Mars crew in their flight quarters would be 18 months in lunar orbit, 6 months on the lunar surface, and another 18 months in lunar orbit before heading back to Earth for a good physical and psychological exam. Both the equipment and the crew probably should to be proven capable in a situation where rescue is possible, before getting launched to where any failure is probably fatal.