How long may learning videos be for young people?

Using a learning platform, it is easy to provide learning videos or videoed material for the learners. The question is if the learners really watch (or even want to watch) these videos. It seems to be obvious that the length of the video stands in direct context of the use by the learners.

Humans have shorter attention span than goldfish, thanks to smartphones

The telegraph, May 15, 2015¹

The length also is correlated to the attention span of people. This attention span can be estimated with approximately 10 seconds. People who are regularly involved in learning or observation of processes may have a longer attention span.

A study (part of the book "Technology Enhanced Teaching", 2016) gives interesting answers to the question mentioned above. Obviously, there is no difference between male and female students.
The preference lies at 5 minutes, 20 % of the students would prefer a length of 2 minutes.

Results or the study (from: Technology Enhanced Teaching, 2016)

Data of the study: n = 72 (18 male, 54 female), age 16 – 18 years old attending a secondary high school.

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¹ http://www.telegraph.co.uk/science/2016/03/12/humans-have-shorter-attention-span-than-goldfish-thanks-to-smart/