The answer is: however long you need to accomplish your goals with the episode. I don't often recommend sticking to a specific length because you do have room for flexibility, unlike a TV show which would likely have programming set for certain hours, and therefore must be required to stay a certain length. I recommend sticking with a range, such as 30-45 minutes, or one to one and a half hours. Short shows are successful, and long shows are successful, too. Either one works, just stay consistent within that range, and keep publishing, and you'll grow your audience!