VoIP Acceptable Delay Recommendations

ITU-T network one-way delay for voice applications Recommendation G.114:

Range in Milliseconds	Description
0 to 150	Acceptable for most user applications.
150 to 400	Acceptable, provided administrators are aware of the transmission time and its impact on the transmission quality of user applications.
Above 400	Unacceptable for general network planning purposes. (However, it is recognized that in some exceptional cases, this limit will be exceeded.)

Clustering over the IP WAN:

The maximum one-way delay between any UCM servers for all priority ICCS traffic should not exceed 40 ms, or 80 ms round-trip time (RTT). 20 and 40 for UCM under 6.1 versions. Delay for other ICCS traffic should be kept reasonable to provide timely database access.
Propagation delay between two sites introduces 6 microseconds per kilometer without any other network delays being considered. This equates to a theoretical maximum distance of approximately 3000 km for 20 ms delay.

Fixed delay values:

Coder delay - 18 ms
Packetization delay - 30 ms
Serialization (64 kbps) - 5