Apple headphones maximum db output?
Extracted form Apple's Store In-Ear Headphone's overview:


  Technical Specifications
  
  
  Frequency response: 5Hz to 21kHz 
  Impedance (at 100Hz): 23 ohms
  Sensitivity (at 100Hz): 109 dB SPL/mW 
  Drivers: Custom two-way balanced armature (woofer and tweeter in each earpiece)
  


If you are using the regular Earphones, although I couldn't find any official technical specs, it's dB output would be lower than the In-Ear's, something like 103 dB.

And for the safe decibels, just remember that:


  
  A typical conversation occurs at 60 dB – not loud enough to cause    damage.
  A bulldozer that is idling (note that this is idling, not    actively bulldozing) is loud enough at 85 dB that it can cause permanent damage after only 1 work day (8 hours).
  When listening to a personal music system with stock earphones at    a maximum volume, the sound generated can reach a level of over    100
  dBA, loud enough to begin causing permanent damage after just    15
  minutes per day!
  A clap of thunder from a nearby storm (120 dB) or a gunshot (140-190    dB,    depending on weapon), can both cause immediate
  damage.