How long is a "rotation" on Mars?

Standard astronomical terminology (which you have actually inverted in your question) is that a planetary revolution is one orbit around the sun, while a rotation is one turn of a planet about its axis. The Warhammer 40K game and novels are generally pretty good about using astronomical terminology correctly; planetary years are described by as revolutions lasting certain numbers of Earth days. So it seems that a "rotation" in the novel was meant to be a Martian day (approximately 24 hours, 39 minutes); this is further reinforced by the fact that seven Martian years is far too long a time to make time a significant pressure.