A string should be dark and moist. You'll know when it's drying out because it'll turn white'ish and fray. If you keep the string waxed it will certainly last a couple years, I used to recommend that you replace the string every year, today I don't believe that's necessary, not a bad idea, but perhaps not necessary. Watch your string for wear, fraying and "weak" spots. Now if internal strings start to break, you many not notice it unless it's suddenly out of tune. The string could be creeping (rubber bands stretch). Measure your axle to axle and compare it to the factory ATA for the bow. A way off measurement might tell you the string creeped. (permanate elongation)