Check for serving separation in any area with hard bends, or excess wear on cable guard going through the slide. If it has cable stops thats another spot that will show excess wear as well.
Most manfactures recommend every 2 years. My experience is if correctly stored away from big temp flux, in a case or out of excess uv light, and your visual inspection shows no torn fiber or severe serving separation/wear then you are good to go regardless of age.
I personally replace annually. I do shoot alot, but my strings sustain most damage from being dragged through brush, frozen, then overheated, wet then dried, riding on a back pack with straps rubbing them and dust and grime, and I carry it by the string. Actual shooting is not hard on them at all its what they are built to do.