When does light run out of gas?
Light is a form of energy (the only kind we can actually see). Energy cannot be destroyed, it can only turn into other kinds of energy.
If there is nothing to stop the light from turning into another form of energy then it will simply carry on forever.
However, chances are that somewhere, at some point, in its journey, light will eventually hit “something”. A lot of that light will bounce off the surface it hits, and some of the energy will be converted into other forms of energy, like heat. Eventually it is possible for 100% of the light energy to be converted into other forms of energy.
But, assuming it is not converted into anything else it will just continue along its path forever