As strings age, contamination, corrosion and normal wear reduce brightness, sustain and responsiveness, and on an acoustic guitar those changes are easy to hear.
How quickly your strings wear depends on how often you play, your environment, your body chemistry and the type of string. Phosphor bronze and Monel strings are both known for holding their tone well because they resist corrosion.
Replace your strings when they sound dull, lose sustain, feel rough or stop holding tune.