If you meditate really many hours a day(example: 8 hours) then duration matters. These long meditations are really beneficial and they forces the meditators to meditate correctly otherwise negative mind states like boredom can take over. But in other cases it is better to do shorter meditations few times or many times a day so it will lead you to be more mindful during the day.
Changing the time of the formal meditation can be problematic If you don't decide to increase the time of your meditation sessions. So doing the formal meditation in the times that you get used to is better.
Doing short meditations puts you in a better position than the vast majority of the humans who don't even have awareness for 1 second a day. But in the long run it is not enough. But If doing short amount of formal meditations lead you to be mindful during the day then it's beneficial.
The important thing is the continuity of the mindfulness during the day. Unless a person realized Nibbana before and attained one of the four stages of enlightenment, it's necessary to practise mindfulness all day long. That's the only way to get results in spirituality.
This video is about short meditation sessions: https://www.youtube.com/watch?v=3XyQBI1ZPJ0