You might need to see an eye doctor if you notice a change in your vision. Also if you notice yourself squinting or having to approach yourself to the objects you are viewing.
Depends on the context. Inter city distances in kilometres, distances around the house or garden in metres, smaller objects in centimetres.
Scientists or specialists would meause interstellar...
In terms of SI Units (Systeme International), which dictate the metric units, scientific calculations involving distance use METRES. So for example, solving an equation like Speed = Distance/Time,...