What is more tangible than your body talking to you in the form of pain, soreness, tightness of your foot, Achilles, calves, knees etc.? Listen to your body and you will know when to replace shoes. If you are unsure, wear a new pair for one match and if pain/soreness/tightness of your lower limbs is much less than when you wear your old pair, it is time to replace your old shoes.
Take the same approach with poly strings when they go dead - at the first sign of any tightness in your wrist, arm, shoulder etc., cut them out. Usually you will also start seeing a loss in control at the same time, but some players are not as sensitive to it. Listening to the body is the best way to prevent repetitive injuries and keep playing tennis daily.