Standard or regulation

ITU-T Recommendation G.114: One-way transmission time

International Telecommunication Union, ITU-T, 2003

Read on October 17, 2026

What it is: The ITU-T recommendation on the delay that telephone and interactive voice connections can tolerate. It gives 150 ms as the level below which most users see essentially transparent interactivity, and 400 ms as a planning limit.
How much weight it carries: An international standard, here in a university-hosted copy of the May 2003 version. It concerns telephone and VoIP calls, so it is a reference point for how much delay people tolerate in conversation, not a measurement of speech recognition.

Facts taken from this source

ITU-T Recommendation G.114 says that if mouth-to-ear speech delay can be kept below 150 ms, most speech and non-speech applications experience essentially transparent interactivity.
“if delays can be kept below this figure, most applications, both speech and non-speech, will experience essentially transparent interactivity” (Introduction to the delay limits)