I'm certainly no expert on this subject, but based on my very general, common-sense understanding of food preservation, and a good bit of hygiene knowledge gained through many years working in healthcare, I'd say they'd stay good for a virtually indefinite period, as long as there is still alcohol present. Alcohol is commonly used as a preservative (think of tinctures, vanilla extract, etc.) and antiseptic (think of rubbing alcohol), and provides an environment that is toxic to most microorganisms.
And yes, the fruitcake reference was another good example! I have no doubt that, centuries after the extinction of humans, there will still be perfectly well-preserved fruitcakes on this planet, especially of the brandy-soaked variety (unless they're eaten by that other indestructible pest, the roaches
).
Rum Soaked Raisins Question 
