i just replayed from scratch to test hearth and home improvements after about a year away. having good working knowledge of the game from a previous playthrough, i beat the last boss in about 100 hours. wasnt rushing, just playing normal speed, gathering plenty of surplus resources like iron, silver, blackmetal, not just the bare minimum.

so, for a new player still learning the mechanics and alt-tabbing to the wiki, etc., maybe 150-200 hours to beat the game and ostensibly make all the equipment (weapons and armor) that you want.

if you want to build and explore, there is really no upper limit. build castles on the highest mountain peaks, or on top of the stone obelisks in the plains. sail around the entire edge of the world. gather flametal from the ashlands (does nothing currently, placeholder metal). build bridges across rivers until you can walk from one side of the world to the other, no boating. find an ocean oasis (tiny sandbar in the middle of nowhere) and build a base on it (hoe raise ground). build farms/automation. hunt sea serpents for fun.
there is plenty of unstructured end-game sandbox content.