>When doing a demonstration for an application how long should it be?
>
>Considering that I don't want to lose the interest of the person getting the demo.

I'm not being cheeky here.

1) ask them how long they want the demo to be
2) ask them what exactly they want to learn from the demo
3) cover points from 2 during time indicated by 1
4) have all of the answers to 2 written down so that if you run out of time, they still will get all of the answers to what they want to know. Also this will mean that you won't have to worry about running out of time.
5) let them interrupt and ask as many questions as they want, this will keep them interested and engaged.