Calibration doesn´t need to be repeated - only the mag incl&decl will change when traveling.
Depends on which direction you are traveling:
- if you travel South -> North (or vice versa) the mag declination will change most while inclination stays similar
- if you travel East -> West the inclination will change while declination stays similar
You can play around at
http://magnetic-declination.com/ to watch the effect.
We had reports of users traveling 1500km without having to change their values. Last year some team members drove distances of 600-700km to the Maker Faire in Germany. Afaik nobody had to change anything...