On 1 world I spent 1606 game days before declaring it completely explored.
https://steamcommunity.com/sharedfiles/filedetails/?id=2720278523

The only areas I did not walk through completely were:
- Deep North : quiet, beautiful and picturesque
https://steamcommunity.com/sharedfiles/filedetails/?id=2663922268
- Ashland; lots of Surtlings. If you want to increase your bow skills, great place. Troll armor and wooden arrows will do (also mined Flametal)
https://steamcommunity.com/sharedfiles/filedetails/?id=2699638836

-Mistland: Dark and very depressing (can't wait for this to be added). Not even worthy of a screenshot yet.