You may notice that all your sittings are lasting for the same amount of time. While you are in the sitting, it might seem like an eternity, or you may not think about time at all, but then the impulse to stop will come. It’s like waking up in the morning without an alarm, but at the same time every day within a few minutes.
Our mind keeps time by habit. So if you want to have longer sittings, the current habit needs to be broken. Habits are formed or changed the same way. Do something once and it’s likely to be repeated. Do something twice and a pattern is formed. Do something three times and a habit is started. So when you are in your sitting and the impulse (habit) arises to stop, tell yourself to stay. You can actually feel the habit pulling you to stop. The impulse to stop will come again, and soon, and you will need to refuse it. The impulse will come a third time, more insistent this time, and you will refuse it and stay. You are now “over the hump” and free to extend your sitting. In concentration meditation, sittings of at least an hour are much more productive.
The Meditation Process 