The question I have asked before is how long should you meditate? There appears to be no right answer, because it can depend on how quickly you can rid yourself of a busy mind. I was sitting this morning and set my timer on my cell phone for 20 minutes, but after only a couple of minutes I had cleared my mind of all thoughts and just listened to my heart beat and breathing. I only meditated for 10 minutes because I was able to reach a calm and peaceful state so quickly.
Then there are other times where I sit for 20 or 30 minutes and may never reach a calm state, and my mind is overwhelmed by thoughts that I just cannot escape. Much of the literature I have read says the optimal time is 20 – 45 minutes, but that doesn’t mean that you cannot benefit from 10 very good minutes as I did today. If you are new to meditation I suggest you start with 10 minutes a day for a week, and then add 5 minutes each week until you get to 20 minutes. Maybe this will not be sufficient on some days and you may need 30 minutes. You need to adjust the times according to how you are feeling and how quickly you reach the desired state of mindfulness.
I am just beginning to add an evening session in addition to my morning meditation, so I cannot really speak to the benefit of this until more time has elapsed. My hope is that I will reap even greater benefits from mediation with a twice a day regime, but we will see.
References that expand upon my own opinions: