Infinity Castle is not literally measured in distance in the story; when people ask “how long is Infinity Castle?” right now they usually mean the runtime of the Demon Slayer: Infinity Castle movie, which is officially 155 minutes, or 2 hours and 35 minutes.

If you were asking about its size in-universe , the series never gives a concrete length in meters or miles; it’s portrayed as a magically shifting, effectively endless fortress rather than a space with fixed dimensions.