herefishy,
As long as the line stayes out of direct sunlight for extended periods and does not get exposed to heat above 85 -90 degrees for long periods they will last a long time. Like uncle Jesse has an old line I was given some fly fishing stuff from along time fly client who past away. It had an old 1960's cortland 333 dt5 new in the box that look just as good as the day it was made. so i guess to answer your question yes they do but you can prolong it by taking care of them. At the shop I guide out of they have all the lines, tippet and leaders on a wall facing away fromt he windows so they are not exposed to too much sun light.

steve