How long does a cycad leaf live?
 

I've recently gained interest in growing cycads that are cold-hardy to my area (upstate South
Carolina, Zone 7). I'm wondering-- how long do individual cycad leaves live? I have two C.
revoluta that have produced new sets of leaves-- one started with five and produced five, the other
started with six and produced fourteen. I know that some cycads produce 2-3 "throws" of leaves per
year, some 1 or 2, and some maybe produce a new set only every couple of years. And yet I see
cycads covered with dozens of leaves. Which seems to imply that individual leaves can live for
several years, else how could so many build up when the leaves are produced so slowly?

Can anyone give me an idea of the typical "lifespan" for an individual cycad leaf?