CDs and DVDs are guaranteed by manufacturers to last 50 years. Keep in mind, that media hasn't been around that long yet, so there's no telling how long they'll last -- maybe more. I mean, if you watch a DVD 5 - 10 times and just keep it as a collector's item, I'm sure it'd last an eternity.
Hard drives are the same, I hear. 30 - 40 years or something... maybe.