I do not believe that length is a big issue at all, but is something that needs to be considered and thought about. Each podcaster needs to determine what his or her goals are for the show – overall format and depth of details covered in the recording will determine the length. You will find that half of your audience will want a short show and the other half will want a long show. So you cannot meet everyones desires.
Make the show length what you think is right for you and consider the value the audience is getting from the show based on the topic focus and segments. Do be concise, but have fun and give some of your personality over time as that creates a deeper personal connection with the audience. Over time this will create a friendship like relationship with your audience that will lead to the acceptance of longer shows. The comfort level you build over time will tend to cause your shows to get longer naturally anyway, unless you are rigid about it.
Always ask for audience participation and talk about emails or tweets they send you. Always think about what value you are offering and is it fun conversation, news or useful tips. All of these have various degrees of value based are your knowedge and talents. Most of all make a show that won’t burn you out to do every week as it is common for podcasters to start a new show and podfade after 6-10 episodes then stop.