Nothing magical about 30 minutes. I've heard fantastic 12 minute sermons, and horrible 45 minute sermons.
I once heard a Jewish Christian preach under a 10 minute sermon and it really was impressive. I didn't learn anything new about the Bible but the though he left me with played over for a few months. I must also confess I've preached more than a few horrible 45 minute ones in my life. Very true. I think if your doing exegitcal sermons 30 is probably a good safe minimum, since your weekly building on each message. I still prefer around 40 to 50 minutes or so.
I guess it depends on how large the passage is and what you are trying to accomplish. I tend to go 25-30 minutes.
I recently preached an 8 minute sermon at a wedding ceremony and was surprised at how well it was received. Think about the sermons by Peter and Stephen in Acts. It wasn't the length, but the content, and the occasion, and the audience etc.