This probably depends on several factors, but my best lens (from times of Squidoo) got about one thousand views in one day and this caused dozens of re-tweets, huge rise in Google's ranking (from number nine to one in less than a week), what all caused further improvement in 'natural' traffic, number of FB likes, pins etc. It also gained several authoritative natural links.
I think this is the right definition of viral - to get enough quality (!) traffic which leads to additional visitors, who will improve all the scores from GA (CTR, bounce rate, etc.). I recently had an article with four thousand visitors (it was Charles Perrault's birthday, what was featured on Google's doodle, and my article was first in my native language), but this bum in traffic didn't give me any material or authoritative compensation. In this case I would not say this was really viral article.