The ITU say voice should be <150mS, however in the real world people are a lot more tolerant. A GSM -> GSM phone call is ~350mS, and very few people complain about that. That said the quality of the conversation is affected, and staying under 150mS is better for a fast free flowing conversation. Most people won't have a problem at 600mS and will have a problem at 1000mS. That is for a 2 party voice call. A large group presentation over video can tolerate more, but may have issues with talking over when switching from presenter to questioner for example.

Simon