Wax your string religiously. Store your bow properly in a climate controlled environment. Check cams, wheels, rollers and guides regularly for dirt and burrs. Do this and you can get many years out of a string if you are an average shooter.
That said if you shoot a few times per week I suggest a new one every couple of years. Not for the fat lip factor, although that is something we all want to avoid, but because it wears inside more than on the outside. Stretch, and torque need to be addressed.