I read, I think, in Tim Ruman's book that Fiber can start to lose it's edge after 6 months at room temperature.
RC I suspect is more robust.
Having said that if I have a box of fiber that is well over 6 months old I'm gonna keep using it as required until I notice that it's going off.
Hey I wish I could buy in bulk and store accordingly but that's not an option for me and I'm not gonna lose sleep over it!