This article on Slate.com reveals what percentage of readers actually finish reading their articles. But their chart is hard to read because it’s a histogram. We can’t see how they lose readers’ attention over time. So I converted the chart to percentage retained over time. As you can see, about 60% of the readers make it to the half way point and the rate of drop is relatively slow, but thereafter they begin to lose the readers more quickly. At the end of the article, they only retain about 11% of the original readers. This is a convenient number to keep in mind as a writer. (If you perform above 11%, your article was more engaging than the average. My last 10 essays averaged 26%.)
I would guess that this is partly because magazine writers commonly employ the “inverted pyramid” scheme where the important pieces of information are presented up front. If they were to employ more traditional structure where the punchline comes at the end, I would imagine this number would dramatically increase for them.