With the right care a quality line can last 6 to 8 years at least, but some modern lines are more chemically unstable and deteriorate much more quickly, not much you can do. Its already been posted about cleaning lines which are one of the keys. Like all equipment a bit if care will extend their life. I have a Cortland 44 that is about 20 years old with no issues at all. Winter off reel storage is another practical idea. If you fish "dirty water" that is waters with a lot of oil and deposits cleaning more often is a must, a variety of chemicals will break down your line. What will really beat it up are rocks and other rough areas that will nick and cut the finish.
Usually when you start seeing small radial cracks it shows that your line is coming close to the end of its life. Chemical plasticizer migration starts happening over time (the stuff that makes a line supple) and can often be hastened by chemicals such as bug spray (a bad one) suntan lotion, gas, excessive heat (like leaving your reel on the dash of your car), or long exposure to the ultraviolet rays and direct sunlight. Check your rod guides for sharp or rough areas, and especially the line guard area as they can also be murder on lines. Stripping line off a reel across a rough line guard can damage a line quick. The other thing I would see a lot when guiding is people who strip off line and have it laying on the ground or rocks while fishing, often stepping on it. Keep the loops in your hands and off the ground.
Here's a classic way to care for lines
On a side note I own and fish silk lines for my bamboo rods that where made in the 20's and have been heavily fished over 4 generations of fly fishers in my family.