When subjected to a muzzle blast(artillery) that has a concussive force or a blast that has screeching sound component, immediate ear damage may occur. Normal gunfire is way below that level.
That's about as dangerous and reckless as a statement can get. Do NOT shoot even ONE shot without ear protection if it can be prevented. The damage is not reversible and I promise you WILL miss that hearing when it goes away. As has been mentioned it is accumulative, so every shot counts whether you like it or not. I have e-muffs on my nightstand. I don't expect to be able to don them in a full-on emergency, but for the I thought I heard something
times, they are a valuable tool, which may help to save your hearing as well as your gun saves your life.