• 3 people looking at the same CAPTCHA agreed on the reading just 71% of the time.
  • Average time to solve a text-based CAPTCHA = 9.8 seconds.
  • 3 people listening to the same audio CAPTCHA came up with the same value only 31.2% of the time.
  • Average time to solve an audio CAPTCHA = 28.4 seconds.
  • Time to solve was even longer for Non-native English speakers
  • The percentage of users who give up in frustration varies, but figures of 20% have been reported.

Via 5 Myths of Password Security citing How Good are Humans at Solving CAPTCHAs? A Large Scale Evaluation [pdf]