Motorcycle.com has an article by Evans Brasfield on hearing loss among bikers. As Brasfield points out, for most street bikes, the real culprit is not engine noise, but wind. The link has some charts that show decibel levels and acceptable exposure times. Also, the writer does a good job of explaining the logarithmic decibel scale.
I have spent many hours shooting, riding very noisy two-strokes, running tractors, chainsaws, and other machines. As a kid, the noise of the vacuum pump in the milk barn was a constant. It never occurred to me that hours of that noise twice a day every day would damage my hearing. My father suffered from extreme hearing loss in his later years. He had to have hearing aids and still could barely carry on a conversation if there was any background noise at all.
I don't expect to get that bad, for a couple of reasons. One is that I am not around noisy equipment as much as I used to be. The other reason is that I am almost obsessive about wearing plugs or muffs of some sort if at all possible. I just refuse to use the trimmer, blower or chainsaw without hearing protection. More and more, I wear earplugs when I run the lawnmower or the tractor, and, of course, when I shoot, especially my handguns.
Despite my best efforts, though, I can tell that my hearing acuity has declined. I used to be a bit of an audiophile with high-end components and speakers. A dinky MP3 player is all I need these days. I can no longer appreciate the fine distinctions that a quality system provides. I can tell muddy bass from clear bass, and that's about it. I can still hear better than I would expect on the higher end of the scale, but I think one of the reasons I like the old music better is that I know how it used to and is supposed to sound.
I am a little conflicted about wearing earplugs on my bike. It seems contrary to common sense to deliberately diminish one of my senses, but I think, generally, a rider depends much more on sight than sound on the road. I do have baffled plugs that will let me hear sirens and horns and other warning sounds while merely reducing the decibels to a less damaging level.
No matter what you do, your hearing is important and should be protected as much as possible. Loud noises, including loud music, can quickly cause irreversible damage.