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.
Most podcasts I listen to are just under or just over an hour in length. It allows me to listen to half on commute into work and half on the commute home, which works perfectly for me, but that could just be that it works so well for my situation.
But I agree with EndlessWaves, the content has to be entertaining, thankfully the ones I listen to are.