Talk for 20 minutes out of every hour and use the rest of the time for reinforcement, skills practice or discussion.
Devote 15% of your time to lecture or presentation, 85% of the time should be devoted to small-group activities and large-group sharing and exploration.”
Adults can listen with understanding for 90 minutes.
They only listen with retention for 20 minutes.
They need to be involved every eight minutes.
So, break your content into chunks that are 20 minutes or less in length, and involve people in those chunks at least once every eight minutes.
From The Trainer’s Handbook for Participative Learning, by Margolis & Swan.