If you've got more than about 24 days worth of music in your "Artists" playlist, then this could happen, I guess -- I could have sworn we did our sums in 64-bit, though, so you'd need about 292 million years of music (I think -- Doug?). What figures do you get when you go into the "Artists" playlist? Does it all add up? Anything suspiciously large (or negative) in there?
You could be being bitten by an old bug, which I thought there was code to fix in emplode -- essentially, the "duration" tag is figured cumulatively, but was occasionally getting stored on the player, resulting in a doubling of the duration every time you looked at the properties of a playlist. This one was fixed ages ago, though -- including code in the sanity checking to remove the "duration" tag from playlists.
If you find any tracks with suspiciously large durations, try "Reset Metadata" on them, and see if it fixes it.
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roger