] In research described in the journal Science last month,
] the team looked at footnotes from scientific articles in
] three major journals -- the New England Journal of
] Medicine, Science and Nature -- at three months, 15
] months and 27 months after publication. The prevalence of
] inactive Internet references grew during those intervals
] from 3.8 percent to 10 percent to 13 percent.

They cite the average half-life of a particular webpage as about 100 days.

In compiling my Kryptos site, I run into this all the time. I find really awesome information and pictures and link to them, but then within a few months, those valuable pictures are gone. So now, I find myself archiving/mirroring anything I find, so that I can re-post it if it vanishes. Does this mean I'm violating copyright? Or saving sometimes irreplaceable information for future researchers? I say the latter, but the time that I spend in conserving other people's work does feel awkward to me at times ...

On the Web, Research Work Proves Ephemeral (washingtonpost.com)