The fast answer to "how long should our podcast be?" is: as long as it needs to be.
The article "IBM reaches shareholders through podcasts" notes:
The two that IBM posted in August were downloaded 10,000 times, Edwards said. The company plans to post a couple a month for the next year. But they will be [...]

If you have never heard of the term Wiki, I will give you a quick overview of what a Wiki is and how it can help you become educated about podcasting in many ways.
A Wiki is a free online resource site that is community owned and operated resource on this topic. The site is maintained [...]

Apple has just released a new version of their very popular iTunes software. This software has approx. 30 million users. The new release (ver. 4.9) of iTunes now offer support of Podcasting. This is big news. By Apple embracing the podcasting medium officially; we can expect to see more market adoption of podcasts.
As users upgrade [...]