Not every meeting can be done in 15 minutes, but for general day-to-day things, 15 minutes is ideal.
Work expands to the time you schedule for it. If you plan a 2-hour meeting, it will likely fill two hours and waste valuable work time.
If a meeting has a purpose, 15 minutes is sufficient and tasks can be made and assigned.
Our brain and attention spans have limits. The act of listening is equally draining as thinking hard about a subject. The more information we are asked to take in, the more difficult it becomes to stay focused.