Light takes about 5.5 hours to reach Pluto from .
If you know the time that light takes to reach certain object, you can deduce the distance to that object using the following formula.
z is the distance.
c is the speed of light.
t is the time that light takes to reach certain object.
so,
t = 5.5 hours = 5.5 x 60 x 60 = 19,800 seconds.