Wow, I love reading a book on the web. Either cut your copy down by 1/2 (no one is going to read the whole thing) or place a disclaimer at the top on the average it will take to read the whole article.
Or ya know, as an beings who can keep track of things like how fast we read, you could simply scroll through the article like i did, go, okay that's about ten minutes. Then decide if you want to keep going. Saying no one would read the whole thing is a bit much to, considering I read every word, and I'm sure others did to.
The indicator you're looking for is the size of the scroll bar. Not perfect, but I think it's pretty silly to ask authors of long articles to write a disclaimer about the article's length when it's right in front of you. Also, this is not exceptionally long for an article, I think the lack of pagination contributes to your perception.