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locked by BinaryMisfit Jan 18 '10 at 8:41
See: Why does the default time range on the 'recent activity' page work as it does? for a discussion.
Roughly, it uses the last time you looked at the data as the start time for the interval. If you go to the page repeatedly, you'll see some information, then no information, and then (if you've been given some rep or comments) some information again.
I always assumed it displays whatever happened since my last visit of the site. If yes, then there can be no fixed time range.