Readings I have been to in bookstores which have sometimes up to 2 hours are more freeform. In that time you can: read, do a little talk, answer questions and sign books. It is a good thing to have a mix of options because you may get a less committed audience, those who stop on the spur of the moment.
Readings at literary cons are a different matter. Your audience is much more interested in hearing you read, and may actually be interested in your work personally. Readings are scheduled for either half an hour or one hour, and you are expected to read for all but about 10 minutes. In the time leftover you can talk or answer questions, but that is peripheral to why you are there. Sometimes the reading will go the full length of time. Author and audience have been known to move en-mass to other accomodations to continue (if the author wanted an hour and was only given a half hour, or if the author is looking for a good place to stop).
The bigger names: Gaiman, Bujold, Martin are scheduled for the hour readings, and people never lose interest. Those authors are also given the biggest rooms, and they still are SRO - and you will have people hanging into the hall (standing), where they can't even see the author and can barely hear for the entire hour. So if you think the audience is going to 'lose interest' then you probably shouldn't be reading in the first place.
I agree with the advice about practicing. One of the readings I went to was by Jon Courtenay Grimwood. It was very good, but he was extremely nervous and skipped around a lot. He noted that in the UK he was only used to reading for 10 minutes and was really not comfortable with the half hour he was expected to read. He is just now being published in the US, so he didn't have people in the audience who knew enough about him, or his books to ask questions. So if you are trying to break-in (as a new author, or in a new market) don't expect to be rescued by questions from the audience, especially at a con - they have heard all the blather about how or why you write and are there to listen to your work. If you do a good solid reading there may be questions afterward about what you read, but there has to be enough of a whole in the audience's mind to come up with them.