I had a discussion with a rep earlier this year about the same thing. His comment was it depends on the quality, use and how it is cared for. I had some that broke down within 12 months. He asked how I was storing it (washed and out of the sun/heat - ok) and what I had been catching on it (barra, spotties, etc on 4lb and used often- hmmm). Apparently if the line is fished at its maximum load a lot, it shorten the life. My heavier stuff 65-80lb is lasting much longer. Guess I need to fish it more at its maximum.
Hope this helps, it is only what I was told.
Steve