That depends on whether it's entertainment or informational.
If it's information then it probably shouldn't be a podcast at all. Speech takes a long time to communicate the same information compared to reading it. Unless it's about something that needs a high percentage of audio or motion then I personally wouldn't bother with a podcast about it because it's just a waste of my time. Generally the answer there is 'as short as possible'.
If it's entertainment then it depends on your ability to keep the audience entertained. Unless you've done this sort of thing before I would start short. Five minutes, maybe ten, and cut ruthlessly.