There's a dive bar in my neighborhood that I go to for the cheap pool. They've got Summit Pale Ale on tap, which is a beer I love, but man, it doesn't taste anything like it does at a good bar. The hop flavors are all muddy and indistinct, and they always have way too much foam coming out of the tap. I can't prove it, but I'd wager that they need to replace their beer lines.
How often should you replace your beer lines? Well, about once a year is fine as long as you keep up with your cleaning. Recirculating BLC through your lines once every two weeks should do a good job of keeping them clean. Alternating with an acidic sanitizer like Star San will also help. If you are cleaning regularly and still notice your lines discoloring, with small spots of beer stone on the inside, it's time to replace them.
Keeping your lines clean is a cheap, easy way to ensure you are serving the best beer. Bad lines can cause turbulence and unwanted off flavors in your beer, so keep it clean!