The main target for an orienteering course is winning time. It goes without saying, then, that courses on steeper maps will need to be shorter to reach the same winning time.
Some rough course lengths are provided here as a target. However, it is also recommended that course setters look up previous events on the same map on Winsplits, and assess whether the course distances of the previous event were too long, about right, or too short for each event. This will give a more accurate guideline for course lengths to be aiming for.