What is the best time length for an online seminar?

What is the best time length for an online seminar?

I plan to give a live seminar that I expect will take up the morning, perhaps from 8:30 AM to 11:30 AM.

But I want to offer this via the web also. It will be live, but wonder if people have different expectations for webinars?

I have been told it should be no longer than 2 hours. But this would force me to make a shorter version of the seminar, and I wold need to take out important information.

What are your thoughts about the length and if I really should reduce the amount of content?

Or, if I take breaks and let people stretch, is three hours of sitting about the same, regardless of where it is?

Thanks