i think it took me about 200 hours to find the merchant on my first world. though a lot of that was spent learning the game and building a good basecamp.

2nd world it only took 30 hours

When your exploring by boat make sure you get close to the shores of new land masses. When you actually enter the new zone enough for things to spawn on shore the game will update your map with the merchant if he is on that landmass. on my first world it turned out I had sailed right by him earlier in the game but was on the wrong side of the water channel between the islands for the map to update with his locaton.