There are many questions on WBSE that cover most aspects of building a planetary system. (some examples below):
But it occurred to me that while a sci-fi story setting might need the complete details of the system, a story set in a medieval or any similar society with pre-telescope or primitive telescope technology would only need to build the parts of the system that would be observable by the inhabitants of participants of the story, or the well known and widespread knowledge of the society that those participants are members of.
So ... How far away (maximum distance) can humans detect planets with the naked eye?
Assume best observable conditions for naturally occurring planets. As far as I can tell from my limited research, this should limit the albedo of the observed planet to about .8 (unless you can give a reasonable explanation of why it should be more or less than that), and the radius of the observed planet should be no more than about that of Jupiter (again, unless a reasonable reason for an exception is given). Assume ideal observing conditions, such as no light pollution, good (perfect specimen human) eyesight, ideal alignment of star and observing planet and observed planet for best lighting of observed planet, etc. Also assume a sun-like star, and earth-like planet as far as atmosphere and other observation characteristics, though human habitability is not a requirement except as it directly relates to human-like observation (a fictional atmosphere is allowed, as long as there is an explanation of how it improves observation, while not entirely preventing complex life in general). Earth-like orbit is NOT required for either planet.
This question has applications not only for general world-building, but can also be used as a basis for calculating orbit times which then apply to things like creating mythologoes, calendars, religious influences, cultural iconography, and much more.