How long does it take an iphone 3 to die completely?

Heres the story, I downloaded this app called cocoppa (stay away!) and it made my phone get this white screen. I try to shut it down but nothing happens. I suppose my only hope is to let it die. Unfortunately, I had charged my phone to a whopping 100% before this operation took place, so my question is, how long do i have to suffer being phoneless before my phone completely dies? The screen doesn't go dark at all, so i assume it shouldn't be that long as it uses more energy being bright.

Added (1). Life safer!

Hold down the home button and the power button until you see the apple logo (up to 20 seconds).

I agree with the Apple logo method, however If you leave it at your desk side, it should take approximately 4-5 hours depending on how worn your battery is.