FDR - Old type
A while ago someone was asking about the "old style" FDR and someone mentioned that they used a "Stainless steel" tape.
Others then asked how long the data would survive on a stainless steel "tape".
Just to clarify, the stainless steel tape is actually a really long roll of stainless steel foil, not unlike your aluminium cooking foil, but slightly thicker, and of course, much stronger and about six inches wide.
There are several "styluses" (stylii ??) in the FDR and each one physically "scratches" a mark on the foil corresponding to the parameters it is recording.
The "tape" was replaced on a regular basis.
So, assuming the box basically survived the crash, that is it didn't physically melt in an extreme fire, the tape and the data recorded on it would last until the end of time, if not longer.
Suffice to say, any number of orders of magnitude longer than any other available technology!