MOD NOTE: I've changed the topic title to an accurate reflection of its contents.
Hours?
It took me ten years to finish the game. Partly because I first got it when I was very young (nine or so) and the later puzzles were too hard for me for a few years; and partly because I grew up with the Mac version, which has an almighty monster of a puzzle in The Steel Mines of Kessel (apparently a very gentle level on other versions!)