http://counter.li.org/reports/uptimestats.php
Five years and two months! However.. was 2.6 even out back then? Perhaps it lies. But wow, that's neat!
According to the data modified time on kernel.org 2.6 came out December of 2003. I'm guessing the one you're looking at is "1892.1 2.6.4-52-smp Intel Pentium III" the 2.6.4 kernel was released in March 2004.