I believe that meditation should not exceed 20 minutes because like all concentration practises, beyond that time it would be difficult to continue to carry it properly. But I don't think there is a standard time, it must be something that each of us must hear. I usually meditate for 8/10 minutes before going to bed because it is one of the times when it is easiest for bad thoughts to accumulate after a stressful day. Meditating helps me create a kind of barrier that allows me to focus on positive things and on myself to have a better quality of sleep and wake up charged for a new day.
Meditating is mindfulness. Any amount of time you invest in this, and what your body tells you it can focus on is great.