What can one expect in the way of life expectancy of fly lines. I
know that if you use them a lot or abuse them a lot (leaving them in a
hot vehicle all summer or dragging them over concrete will shorten
their useful life).

I have a 4 weight Cortland 555 which I've had for one season and took
reasonable care (no long term heat exposure or concrete). I fished
with it yesterday and it looks brittle with tiny cracks in it.

I thought lines should last at least a couple of years under "normal"
circumstances?

Any insights?