To make sure your iPad or iPhone screen doesn't turn itself off while displaying a survey, you should disable the Auto-Lock feature on it.

Here's how you do it:

  1. Open Settings on the device. 
  2. Choose General in the left menu.
  3. Choose Auto-Lock in the menu to the right.
  4. Choose 'Never'.
This will prevent the screen from turning off on its own.  If you want to make it so that other people cannot turn the device off or switch apps using the home or power buttons, you'll need to enable Guided Access Mode.