One light hour is the distance that light travels in one Earth hour.
In 2346, the USS Intrepid was 42.7 light hours from Khitomer when the starship made its first sensor contact of the planet. (TNG: "Sins of the Father" display graphic)
In 2367, Lieutenant Commander Data reported that there was an M-class planet approximately three light hours from the position of the USS Enterprise-D at that time. (TNG: "In Theory")