Dear scottrum,
I change out a fly line when it gets damaged. Sometimes that happens accidently like when I step on it while wade fishing. :-(
Most of the time I just wait until the tip section starts to get rough and/or cracked and dried out before I replace it. That can take years depending on how often you use a particular line and how much care you take when cleaning and storing it.
As far as maintenance is concerned occasionally I will clean the entire line with warm soapy dishwater, rinse it, and then towel dry it before I treat it with Glide or some other similar product.
During the season I try to clean the line with one of those little felt pads you get with Cortland lines before I start fishing each day.
Doing those things seems to make my lines last a good long time.
Regards,
Tim Murphy 🙂