This depends largely on advances in propulsion. It's convenient to launch to mars every 2 years from a fuel efficiency standpoint, but there is no reason why we can't go there at any other time, other than current propulsion limits. At a constant 1G acceleration (including a flip over at the halfway mark for equal 1G deceleration), even at its farthest distance from earth, we could reach it in under 5 days. You even get artificial gravity as a bonus.

https://en.wikipedia.org/wiki/Space_travel_using_constant_ac...