celgins - 4:26 pm on Feb 28, 2010 (gmt 0)
I think the term "long" is relative. Each site owner is different and I have no idea if any web studies describe what "long" means.
For me, when a user has to scroll the entire height of their screen to read the remainder of the article, the article is long. I'm not saying this is a reason to split the article into pages, but users seem to get cranky when they have to scroll a lot.
News organizations know this, which is why you may see an 12-paragraph article split into three pages (with several 300x250 ad blocks interspersed to fill in space). For the most part, each page fits into a 768-pixel height screen and scrolling is minimal.
But like koan noted, many users who see "Continued on Page 2, 3, 4" have no intention of reading that much--even if they didn't have to scroll very far to reach the end of page 1.