If the Biblical Sabbath is only 12 hours long, how long is you Sabbath if you live in the Arctic?

Answer: In Bible times, every day was always 12 hours, whether in summer or winter, because on a sundial, every day has only 12 hours.  A standardized 60-minute hour did not come into being in the Western world until the Renaissance and the development of mechanical clocks.  When one uses a sundial, there are only and always 12 hours in every "day."  A sundial can only be used when there is sufficient light to cast a shadow.  The light part of every day is broken up into 12 segments.  Thus, the individual hours in summer are longer, and the individual hours in winter are shorter.  There were four "watches" in the night: 1) sunset until halfway to midnight; 2) halfway to midnight until midnight; 3) midnight until halfway to sunrise; 4) halfway to sunrise until sunrise.  Again, the length of these watches varied, being shorter in the summer and longer in the winter.
For those who live in the Arctic Circle, keeping the Sabbath and guarding the edges of it would mean being completely ready for Sabbath when you go to bed the night before, and keeping Sabbath from when you wake up until when you go to sleep.