There are documents hundreds of years old. Admittedly, most of those are currently protected, but the high-quality materials required for rune paper do not fall to time alone. Heck, I have plenty of *cheap* paper that is old enough to vote and perfectly usable.
Of course, it can be destroyed by fire, magic, or being eaten by a troll. This is no more relevant to the reusability of rune paper than pointing out that a wand can be broken over one's knee is relevant to the question of how many times a wand may be recharged.
"papyrus books and documents had in ancient and medieval times a usable life of hundreds of years." -- taken from a longer quotation at
http://www.historyofscience.com/G2I/timeline/index.php?id=3121If you use vellum, well, "Lasting in excess of 1,000 years—Gregory the Great, Pastoral Care (Troyes, Bibliothèque Municipale, MS 504), for example dates from about 600 and is in excellent condition—animal vellum can be far more durable than paper." according to
http://en.wikipedia.org/wiki/Vellum.
In short, barring accident, the lifespan of a written document is expected to be longer than that of a professional adventurer.