You should wear a hearing protection device whenever you are exposed to noise that is 85 decibels or greater for an 8-hour period of time (Time Weighted Average). 88 decibels for over 4 hours is the next cutoff. 91 decibels gets you down to only 2 hours. Each 3 dB rise cuts the time in half that you can go unprotected.
The amount of damage caused by noise depends on the total amount of energy received over a period of time, the louder the noise the faster it causes damage. A noise 3 dB greater has twice the energy output and causes the same damage in half the time.
You must also wear hearing protection if there is a "peak noise" of 140dB or greater.
Noise is a fooler. It can sneak up on you.
I pity the poor fool teenagers that ride around in those cars playing that stereo that shakes buildings two blocks away. They will be deaf someday.