Well, I use my line a lot. I really only have 3 lines. The most used (5 wt) gets used on all my 5 wt rods, I move the reel around to the rod I plan on using. The 6 wt line is mountain brookies only and mounted on the spare spool to the 5 wt reel, and the 7 wt on a LA reel that sees only occasional steelhead and bass duty. The 6 and 7 wts get used a lot less than the 5 wt.
Maintenance: Every winter I clean and treat all my lines with zipcast. I may do the 5wt once in the summer too, if I remember or notice dirt, but the winter cleaning is all I'm reliable for.
Durability: 2 years is typical for the 5 wt before replacement. It's a Wt-Fwd line, so it needs replaced. The other two lines I haven't had to change yet, but they're only a few years old. The steelhead line is a DT so it'll just get turned around. The brookie 6 wt line is also DT, but I actually cut the thing in half, so the other half is collecting dust, but ready for use when its needed.
As a side note, its not just number of hours of use. The water plays a big part. Limestone streams seem, at least to me, to dirty a line much more quickly than freestone streams.