I listen to several podcasts on a regular basis. A friend introduced me to the “Hardcore History” podcast a couple of years ago. These very deep (read that as very long) podcasts produced by Dan Carlin and come out two or three times a year.
Yes, only a few a year, but then each podcast ranges from 3.5 to 4.5 hours in length. The most recent episodes can be downloaded for free, with older podcasts available for purchase (About $2 each).
The most recent eleven podcasts are available on iTunes. These podcasts are well produced and provide a deep look at each topic. Most subjects span three or more episodes. The latest trilogy of podcasts, “King of Kings”, deals with the the Kings of Achaemenid Persia.
If you like history, I strongly recommend these podcasts.