There’s no set time. Tubes will get hard and loose their flexibility. You definitely want to change them then. You may also start to notice you are getting feedback, when you weren’t before. You’ll want to change them then. Look for cracking, tiny splits, etc. probably the norm is to change them 2 or 3 times a yr, but that’s up to the individual. Actually you can just change them whenever you want, but it’s not that necessary. If you feel the quality of the aid has deteriorated, sound unclear, not as loud, etc, then I would probably change the tubes and see what happens.