Bruce Richards, who was with Scientific Angler as their flyline man for many years, told me that if you bought a quality line, and he didn't say Scientific Angler when he said it, just GOOD QUALITY flyline, it could last a long, long time, as long as you took good care of it.  He actually said you could hardly wear today's flylines out, but then he stressed that cleaning and caring for the lines were the basis of long flyline life.

Ray