Wow, came across this from the MentalFloss blog, which in turn is from Answers.com.
(update - which is actually from Wikipedia)
"As of 2006, Pink Floyd’s Dark Side of the Moon has been on the charts for over 1,500 weeks, or just about twenty-eight years. The album spent 741 of those weeks on the Billboard 200. The other weeks were spent on the Top Pop Catalog Albums chart. Its closest rival is Bob Marley’s Legend, checking in at over 800 weeks (Billboard 200 and Top Pop Catalog Albums combined)."