Actually in my country, the recommendations are not more than 8 hours exposure at 85 dB to prevent irreversible hearing loss. It is kinda hard to know what is 85 dB when you are plugged into an IEM, though there are some rigs you can get invest in to measure SPL objectively with a dummy head.
I'm in my mid 30s now, been playing piano in a band for 15 years and only in the past 7 years or so have I used IEMs/closed headphones to protect hearing. Prior to that, we were not aware of hearing protection, and were young and naive and just jacked up the amps to have more fun. I think most band practices/concerts, the SPL could hit 100 - 120 dB, and I was always placed next to some subwoofer amps and full set acoustic drums and I had to jack my amp higher to override the drum and house volume. Some days I had ringing in my ears or had muffled hearing (as
@MonoJon said, temporary threshold shift) after a band session.
I did an audiometry test last year and found out I had very mild high frequency hearing loss, semi consistent with age, and likely from all those band sessions when I was younger. And the audiologist said it was irreversible. For normal conversations and music appreciation it doesn't affect me, but hearing loss is irreversible, so please protect your hearing as
@MonoJon says. I wished I knew about the importance of hearing protection when I was younger.
Nowadays, for travelling and band sessions, I am very careful about hearing protection. In particular, I only use IEMs with isolation of 20 dB and above, as these allow a lower volume of music to be played (and also lowers external noise from entering). Even at home, where it is more quiet, I try to resist the temptation to jack up the volume to get more details/kicks. I'm checking out some new tech for bone conduction earphones, which bypasses the eardrums, it's quite nascent and the sound isn't as good as normal earphones, but maybe in the future it can develop well enough to hit the same quality as conventional earphones/cans.