Dave I think it all depends on how you take care of your bow and your visual examination of strings along with you knowing the way your bow shoots for you.
I replaced a set of string with Winners Choice 1 year after I bought a new bow (which was not necessary, didn't do my homework) , now I keep the strings waxed and inspect everytime before I head out to shoot. The funny thing is, I was still shooting a PSE bowstring Baby G force for a while and the strings and cables were never replaced and it still was accurate.
I recommend that if you do a visual inspection for frays, burs, and so forth along with good bow maintenance you will know when its time to replace the bowstrings
Just my .02