I would say two or three things, off the top of my head:

How many people outside the culture know about it, and how far those cultures are from the originating culture (ie, do naked, stone-tool-using tribesmen in the Amazon know who Darth Vader is?)

How long it's lasted. Though you have the corporations deliberately keeping things in the public eye nowadays, so that's not a very organic metric, at least, not any more. But then, there's a lot of books that we only know about because schools pushed them on us, too. Basically, do people still genuinely care about Star Wars two or three generations down the line? Do the grandkids or great-grandkids of the original audience like it as much as their grandparents did?

Also, perhaps how many other parts of the originating culture it's affected; when the original Star Wars first came out, it basically introduced us to movie merchandising and toy tie-ins, which affected the Saturday morning cartoons that came after it.)