As an antsy person, I want the benefits of meditation in a short time. That is not an approach recommended by Buddhists of yore but I am not a Buddhist, yore or today. Clearly, daily training of my own consciousness of what I am thinking of and how I am thinking and feeling has tons of benefits but what is enough time per session.
Ok, take a look at "QR: The Quieting Reflex" by Charles Stroebel, MD. His answer: 6 seconds. Can be done waiting for a green light or a phone to be answered.
Take a look at "Search Inside Yourself" by Chade-Meng Tan, the Google engineer who started the Google program in meditation. Tan says that he and his little daughter do 2 minutes together. He says that is all an engineer like himself can manage.
Take a look at "The Relaxation Response" by Herbert Benson, MD. He recommends 10 to 20 minutes, once or twice a day.
Finally, B. Alan Wallace, a well-known authority on Buddhist matters, says that Indian and Chinese ancients, working independently, arrived at the figure of 24 minutes for optimal results.But some priests and devotees use several periods a day.
This matters for at least two reasons. Beginners often find the practice of sitting still and attending to their breath or a visual target unsettling and difficult. So, if the period is short, more people can learn the practice. Second, if the period can be short, it can be fitted into a day more easily and is therefore daily meditation is more likely.