Ideal Length Of Time Of An Online Presenation

What is the ideal duration of an online presentation (technical subject matter, not-interactive)? Some folks say 5 minutes is enough, others want it to last at least half an hour. We are putting this question out on the web to find out more from the audience. We welcome your comments and responses. Just click on the number box on the left hand side of this text box and drop us your comment. 

We have a bunch of interesting presentations that we wish to bring to the internet. These presentations are free, no doubt, but at the same time we do not want them to take away too much of your valuable time either.  We appreciate your cooperation in this matter… Thanks much, Jorden de Bouve (business developer)