The short answer is that consistency (of practice) is more important than duration (per practice).
Various studies recommend 20 minutes of mindfulness meditation daily to enhance brain functions and overall physical and mental health. However, you have to consistently monitor yourself to decide what is best for you. A short 3 minute session may make you more focused than a 15 minute one or vice versa, there is no one correct duration for everybody. The key here is that any amount of meditation is better than none. Remember to do it every day, even if only for 5 minutes. (Source - "How long should I meditate?" by Mindworks)