Research for Video Length for Math Videos

I’m looking for any references that exist for what a good video length for an online math class should be.

I am aware of these three papers but these are basically only for MOOCs – I’m looking for something in a mathematics class that’s required – Ideally measuring long term learning but at this point anything is welcomed.

• Effective educational videos By Cynthia J. Brame, CFT
  https://www.lifescied.org/doi/10.1187/cbe.16-03-0125

• How Video Production Affects Student Engagement:An Empirical Study of MOOC
  Video – Guo et al., 2014
  http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.714.195&rep=rep1&type=pdf

• Understanding In-Video Dropouts and Interaction Peaks inOnline Lecture Videos

  • Guo et al., 2014
    https://dspace.mit.edu/bitstream/handle/1721.1/90413/Miller_Understanding%20in-video.pdf?sequence=1&isAllowed=y

EDIT: I’m looking at the university [undergrad particularly] level. Yes I agree that static videos usually aren’t best for learning (but say for algorithm demonstration; I find a video to be vastly more effective and efficient than a long prose describing the algorithm as both a learner and as an educator).