Shouldnt this be "how far is a light year"? light year is a meausure of distance, not time.
How long is a meter?
I think you're mistaken. The question actually asks for how long it is distance wise. For instance, can you say exactly what distance constitutes an AU or a parsec? Most people can't.
It's far more common in my region to say "how big" or "how far" than "how long" with distances. ("My region" is Edmonton, Alberta, Canada, where I earned two physics degrees, so I've heard this exact question a lot.)