Ouch. 15 minutes is the rule. 30 minutes is way too long. I was in only one team that followed the rule, and after 15 minutes people would start leaving the room regardless of whether others are talking. If someone had a habit of speaking too much, he was coached.

When we were on site, we would try to schedule the room such that someone else had a booking at the 15 minute mark - so you had no choice but to keep it in that window.