Today we have the answer thanks to several users who watched a few livestreams of players clearing the game. There are no spoilers in this article but the length of time required to defeat the final boss should give you a better picture of how much content there is in World of Light.
One of the players who have cleared the story managed to do so in 16 hours, and they are about 66% complete. Another streamer finished the game in 15 hours, but did not reveal how much of World of Light was completed.
Based on the stats above, we estimate it would take on average 15 to 20 hours for most players to reach and defeat the final boss. It will take about 20 to 35 hours to fully 100% clear World of Light.
What do you think? Let us know in the comments!