If someone has asked you to come to a meeting and you have any kind of power or authority over them, my advice would be to ask them to tell you exactly what the agenda is. If there's no agenda, it's firm and concrete. Tell them to postpone the meeting until they have a firm agenda. Next, ask them what the start time is and what the end time is. Ask them to really nail it down. That can help with meetings a great too otherwise you can just sort of show up.

People start chatting, there's a doughnut in the corner. A few pleasantries are discussed, ideas are bandied about next thing you know, 90 minutes of your life gone.