Do you often wonder how long you should meditate for?
Is it 20 minutes twice daily, or once-a-day for an hour? What about ten minutes in the morning?
I call this the "optimal dosage question" and it gets asked, pretty much without fail, whenever the topic of meditation is being discussed. What's often behind it is a desire to know, "What's the minimum I need to practice in order to get results?".
It's a Western question, with a Western mindset.
To put it simply, we are all different. Neuroscience knows this. Psychology knows this. Turns out even the Buddha knew this. The Abhidhamma (a Buddhist psychology text) is explicit in clarifying that different people will respond to different meditation techniques and should be taught according to their individual needs.
Science is still at the very beginning of understanding how different individuals respond to different techniques and in what dosage. Of the research we do have, positive changes in brain functioning and wellbeing have been reliably documented in studies employing a dosage of 20 minutes daily. That doesn't necessarily mean that you should limit your practice to that, however. Nor does it mean that a 10 minute practice is a waste of time.
Let your experience be your guide. Look to research to inform your practice and turn to your meditation teachers for advice, but study yourself and trust your experience. Blind faith in a method and dosage that may not necessarily be "optimal" or even realistic for you, can lead to frustrating and even disheartening experiences.
Keeping a meditation journal can be helpful in this regard and serves as means of exploring and having a better understanding what works for you.