like anything in CG it depends on what model your setting up, if you have reference images or sketches of the scene your building then we can help. Things viewed in the distance need less detail, mid ground need some detail, and close up needs a lot of detail. Detail effects your rendering time.
Depends on whether your out to render frames to create an animation or render a still image, spend more time researching the idea, the reason why is because you may as well create a wicked render than one that doesn't turn out the way you want
