Denis,
Re duration- it depends on the app and the audience. If you need a demo longer than about an hour, "the agenda" helps audiences focus and helps you stay in control of your demo. So consider distributing a printed agenda at the start, breaking the demo into 55-minute segments with 10 minute bathroom breaks at logical points selected by you.
Also: "A change is as good as a rest." That's why panel sessions are less tiring than lectures- different styles and viewpoints. So see if you can inject some humor. Examples are good- especially examples that will be immediately recognisable in the listeners' experience and in which they can see the difference made by your application. That's fun for them.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
