Many Linux distributions over the years have had a problem where disabling the distro's screen save and power options do nothing. Raspbian doesn;t even have the option to put it out of screen save mode, so at rather annoying intervals the screen simply blanks. Fortunately the fix is exactly the same as it has been for years.
The quickest and simplest way to stop Raspbian going into screen save mode is:
sudo apt-get install xscreensaver
Once it is installed, you should find a new Menu Item under Preferences called Screensaver.
Just open that and select Disable Screensaver from the Modes dropdown list. Or, if you prefer, set a slide show or something. At least you will be able select the time between screen saves.