I’d like to start by first stating that your hearing once damaged does not heel itself. If you cause and keep causing significant damage you WILL develop tinnitus or worse may end up losing your hearing all together.
Experts say that hearing damage occurs at sounds louder than 85 decibels for an extended period of time (a phone rings at around 70db). I believe this is estimated at more than 6-7 hours. But an average drum stroke kicks out around 125db with cymbals being a tad higher. To put this into context it’s the same as An Ambulance siren, pneumatic drill, heavy machinery or a jet plane on ramp. Hearing damage will start to occur after around 4-5 minutes without protection if exposed directly to this. This isn’t to say that after that time you will notice it. The build up of damage could take years before you will notice it happening. But when you do it will be too late.