I don't know how long it will last, but right now you can get the Big Bach Set from eOne Music (Vanguard Classics) for 99 cents. It's right at 9 hours of music and the only negative review is complaining about the download time (always an issue with over a hundred tracks, but so much better now using the Cloud Player/Drive).Just in case the first one sells out, I also turned up another album, Bach - 100 Supreme Classical Masterpieces: Rise of the Masters. It's slightly shorter (at 6-1/2 hours), costs double (all of $1.99) and is produced by X5 Music Group, a label with a number of other good quality, low cost compilations available in the Amazon MP3 store, including a $1.29 Romantic Classical Music for Valentine's Day album.
Another 99 cent choice is Classical Music for the Reader 3: Great Masterpieces for the Dedicated Reader from Cobra Entertainment. It's only about 90 minutes, but that also means it won't eat up all the space in your reader when you download it.