I got it directly from the instructions which came with the GFCI outlets I installed in my shop. I don't have the booklet right now, but it clearly stated something to the effect of, "This unit is designed to work for blah blah. When it fails, it must be replaced blah blah...".
Here's an article which specifically mentions GFCI End of Life (EOL):
http://www.xantrex.com/web/id/2161/docserve.aspx
Note:On some of the GFCI outlets, the End of Life (EOL) is not displayed via LED. EOL is reached
when the RESET button no longer engages (or does not stay pressed in) and GFCI output power is
denied. On others, it is the combination of LED and RESET button position that reflects EOL. For
example, on Pass&Seymour 1595-W, if LED is on and RESET button does not stay pressed in, the GFCI
outlet has reached its EOL.