It all depends on your content. The world average for podcasts is 35 minutes, but 20-minute podcasts are also quite a lot. The books take a few hours, but if you want, you can do 20-30 minute readings in chapters and publish them that way. It is said that 8-12 minutes is ideal in the stories, but there are also 3-minute children's stories. You can split long content and turn it into series.