Light is a form of radiant energy which takes time to propagate from place to place but cannot properly be said to have a speed. That radiation originates from a specific set of time space coordinates that are independant of the motion of the radiating body. There will be a time delay between the radiant energy leaving the source and any effects it brings about. This question appears to have a simple answer. Light lasts as long as it takes to get there.
Light is a form of radiant energy.
That is to say it is energy that originates with a source. There is evidence, the spectral displacement that astronomers observe, that the energy of light does decay over time. The longer light travels the lower its energy when it arrives. There is also some evidence that light can gain energy from gravitational fields and have more energy on arrival than when it left. Red light is of lower energy than blue light, so a loss of energy is a red shift and a gain a blue shift.