In the workplace, OSHA mandates hearing protection at 85 dBA TWA (time weighted average) over 8 hours, and has a PEL (permissible exposure limit) of 90 dBA TWA over 8 hours since 90 dBA is roughly the threshold for start of permanent hearing loss. A good summary:
https://www.osha.gov/SLTC/noisehearingconservation/
At my company, we require single hearing protection at 85-89 dBA (typically using Howard Leight NRR 33 dBA plugs, same as DaveM posted above) and double hearing protection at 90+ dBA (typically NRR 33 plugs AND earmuffs which provide NRR ~30 dBA).
A helmet that transfers 90 dBA at 60 kph is
very noisy and is at risk of hearing damage
in only 8 hours. For every 3-5 dBA increase above 90 dBA, typically you halve the time to damage. (90 = 8hr, 95 = 4hr, 100 = 2hr, etc.) So a 110 dBA noise can do damage in about 1/2 hr.
Regarding NRR (noise reduction rating): plugs or earmuffs are only able to provide their 'NRR' claims IF they fit tight. I personally wear NRR 33 earplugs at work, and I've been tested while wearing them to find out that with careless (incomplete, improper) insertion, they provide only NRR ~20. Conversely, if I insert them properly and let them expand (become tight and almost uncomfortable), I can get nearly NRR 30 from them. And when they're tight, they lead to sore ear canals in a day. But it's worth it.
To Reginald: There are sound isolating earbuds that work pretty well. I get about NRR 20-25 with a set of Etymotic ER6i earbuds, after finding the tightest/best fit seals for my ear canals. The Etymotic MC5 claims NRR 32 to 42 (I doubt they are that good, but that's what the literature says). You can find them on Amazon for $59.