Yes, it is an astronomy unit and That's what I meant by 'technically'.
But in general use, There is no harm to use the term to express something extremely large, it could be either time or distance.
Ow..Technically...
Literally
http://www.yourdictionary.com/light-year
a very great distance, amount of time, etc.: usually used in pl.: light-years ahead in basic research
Read more at http://www.yourdictionary.com/light-year#cb8QPqQfT9bVB3yX.99