In my novel, Shifting Mars’ Sands, set 200+ years in the future, I rely on the ability of people traveling from Earth to Mars in 30-40 days, depending on Earth’s relative position to Mars. I didn’t want it to be too long, so no one would want to do it, but not too short, so anyone could take a long weekend trip, either.
For the purposes of my novel, I needed Mars to develop a cultural split from Earth, and I figured that time frame would enable such a split.
Now, I see an article where a rocket engine is being developed to allow travel to Mars in 39 days. My first reaction was one of justification–it can happen. I’m left, however, with the feeling that 200+ years from now, 39 days to Mars’ll seem ridiculously long.