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The magnetism on a particle used in a magnetic medium lasts forever. It is a single domain with a permanent magnetic charge. The signal on a recorded magnetic medium, however, depends solely on the pattern of the billions of magnetic particles. Recording rearranges their magnetic (not physical) orientation to a particular pattern. Erasing randomizes their magnetic orientation so that no pattern is evident.
On another subject in this thread, the protective lacquer on any optical disc makes no difference what color it is, as long as it seals the edges of the disc and prevents air or moisture from reaching the mirror layer. Sputtered gold as a mirror layer does not oxidize, so it is superior to silver alloys or aluminum only in case the lacquer is defective and allows sulphur-bearing air to reach the mirror layer. In other words, gold is superior only for CD-R discs that might be damaged or defective. Gold reflects less than silver alloy and must be relatively thick in order not to modulate the reflected laser output.
For DVD use, gold is inferior to silver alloys in writable optical discs because the specifications for DVD+/-R were designed for silver alloy. I have never found a DVD+/-R disc with a gold mirror layer to have initial PIE errors lower than about 50, while the best silver alloy DVD discs could have PIE errors in the low single digits.
The average expected lifetime of a well recorded DVD+/-R disc is about 52 years according to the most accurate environmental tests done on them. 95% would be readable after 39 years with proper storage. CD-Rs using phthalocyanine dye would be readable in 125 years. The weak point for both discs is the dye used, not the mirror layer. The quotes of 100/300 years of lifetime expectancy for DVD/CD recordable discs are bogus because the testing was too short and used only a single stress factor--heat--instead of the double-stress tests using both heat and humidity.
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