How long should I train each day?

Just like with physical exercise, it's best to slowly build up duration. Science has shown that 10 minutes on a regular basis delivers proven benefits, but you might want to start at 5 minutes and build up from there.

If you have both the time and the inclination to practice for a little longer, that can be a very positive thing. Most people feel challenged at first, but then after a few days of adjusting, they can't imagine sitting for less. 

One thing that changes as you sit longer is the degree of clarity you experience. It takes a little time for the mind to settle down and only then do we see its contents clearly. This process can feel a bit like a magnifying glass or microscope slowly coming into focus.

At the end of the day, quality is more important than quantity. Even practicing one minute of awareness is valuable. Pick a duration that works for you and build a regular practice. Without putting any pressure on yourself, increase the time when you're ready.