What is the exact walking distance colonists can travel between domes?

The maximum length of a passage is 18 tiles. I'm not 100% sure if that includes the end spaces or not, so it might be a reliable maximum of 14 hexes between domes (+1 for each entryway and +1 for each wall hex they build through). Colonists will relocate any distance as long as there is a fully connected network of domes and passages. (They'll only work/seek services 1 passage away, but they'll migrate any distance.)

I don't know if they'll even bother walking outside to migrate, but if they do I STRONGLY suspect they won't go further than the building radius (displayed as a highlighted border when selecting the dome), same as they would for work.