This is one of the most common questions i get asked and in all honesty i feel there are two ways to answer this question, So here we go.
The first answer addresses the issue of sustainability, If you would like to maintain your practice of meditation i feel that it is better to practice for a smaller amount of time each day. 10 to 15mins as a minimum is achievable for most people. Some of my students just don’t feel they have the time in the morning due to the chaos of the family waking up. So i have even suggested that they do their 10 mins in the car at the parking lot at work. A small amount of time each day is achievable for most people and studies show that even 10 mins a day can have an effect on your mood and sense of wellbeing.
On the other hand i do see the value of prolonged practice as i know it does take time for the mind to authentically calm down. Also in longer sessions you have the opportunity to spend more time with your mind. We find in retreat settings that emotions can take time to come to the surface. Even on day 3 of a retreat you may just be starting to fell past emotional trauma surfacing. So longer sessions do have a correlation to the healing effects of meditation.
Ultimately with all thing in life more you practice the better you will get.