On a treadmill.
I’ve heard it in casual conversation, as in “I’ve been waiting for light years!” but not really in science fiction. Even Star Trek TOS, whose writers made so many major boners–amplification to “one to the fourth power”, anyone?–always got light years right, as far as I can recall.
I remember at least one mistake using light-years as a measure of time in DS9, but it’s been so long I can’t tell you anything else about it.
Damn you! I was going to say that!
But that having been said, I’ve always wanted to hop a ride on the Millenium Falcon and run out about 13 and a third parsecs from Earth and point my ACME super duper telescope back to see if there was anyone on the grassy knoll.