If you're lucky, it'll be for as long as possible.
And it'll most likely be worth every second. I've been to a couple of seminars and they are usually all different in their hours. You can figure on at least a lunch break during the day. Typically, you'll have a morning break and an afternoon break. But not always.
The more time you get on the mat, the better it is. Pick up as much as you can. Bring a very big empty cup and enjoy every moment.
As for content, that depends, too. Some seminars have just instruction, some have demonstrations, some have lectures, and some have both or all three. Usually, though, there is either a flyer or a web site that details the seminar. I'd look for that to see if they have any more info.