The norm seems to be a limit of 3, 4 or 5 minutes per person, regardless of how many people wish to speak. Public participation is merely an opportunity to connect with residents before the main business of the meeting kicks off. It is never intended to be a half-hour session. There is no requirement in standing orders for the council to say or do anything in response to your comments, which can make the process completely meaningless if the council so chooses, and many do.
We also allow people to submit agenda items beforehand under a Residents' Issues item, either directly, or by contacting a member of the council who will present the item on their behalf. It works much better than the public participation at the start of the meeting, as we have time to work on items before the meeting starts and can usually resolve them with minimal debate.