OSHA defines what noise levels warrant hearing protection to assure safe working conditions. Excessive noise is generally defined as exposure to 85 or more decibels of sound over an 8 hour period. But if the ambient noise exceeds those levels, the amount of time until damage can occur will decrease.
Hearing protectors must be tested and approved by the American National Standards (ANSI), and all devices must be labeled with a noise reduction rating (NRR) to quantify their effectiveness. The higher the NRR number, the greater the capacity for noise reduction.
Even though NRR is expressed in decibels (dB), it is important to understand that hearing protectors DO NOT reduce the surrounding levels by that amount.
To calculate the actual noise reduction, take the NRR rating, subtract 7, and then divide by two. So if you have a piece of machinery operating at 110 dB with hearing protection rated at 33 NRR, the reduction would look like this:
110 dB - (33 - 7) / 2
110 dB - 13
= 97 dB
You can then use the chart above to determine if addition noise protection is necessary.
There's obviously little downside to being overly conservative when it comes to protecting your hearing. I tend to use it even if I don't absolutely need it; it's just a good habit.
If you are occasionally working with something particularly loud, you can double up the methods of protection… but if you do this often, it is best to just buy one, convenient piece of protection that will handle those levels.
But if you are layering, it is important to understand that layering noise protection is not additive (i.e. 33 dB + 29 dB ≠ 62 dB reduction). At these ANSI-rated levels, you typically just take the higher level and add an additional 5 dB of noise reduction. In the case above, you'd end up with an effective rating of about 38 dB NRR.
If you are concerned about blocking out the "normal" noises as a matter of safety (communicating with people, hearing alarms, etc), then you might want to consider investing in some electronic ear protection (search). Devices like these will provide an effective reduction of loud noises while allowing (or even amplifying) the normals sounds you might want to hear. That's pretty cool.
I use electronic hearing protection almost exclusively. When I am working with loud equipment (or things that go <boom>), I want to be able to hear the things going on around me. When the equipment goes off, I don't have to remove my hearing protection or remember to put it back on again. When it starts to get noisy, it's just there. And I like to assure that I can continue to communicate normally with the people or things around me.