I wouldn't expect plans or stanzas to decay, and even skills shouldn't decay below, say 75% to 80%. If you look at the real world you don't comepletely forget how to do something, but you do lose the high end edge. So if a skill requires a certain level of practice that you no longer have you get a substantially increased chance of failure. In game this does not mean you have lost the stanza but that you can no longer use it.
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Remembering Tyneetryk
Phaedreas Tears - 15 years old and first(*) of true neutral guilds in Atys.
(*) This statement is contested, but we are certainly the longest lasting.
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