Extending the time Ubuntu 12.04 stays open without making the screen go dark

[Jonesy]

On Thu, 12 Feb 2015 13:54:34 -0500, Don Parsons wrote:
> Hello.
> I would like to have the screen stay live longer than the present few 
> minutes (or go dark for a short time and then open up again repeatedly 
> and automatically). I find the present need to keep signing back inm 
> when the screen goes dark, rather annoying.

  ... and you want to do this for _which_ desktop environment?