Settings->Theme->Screensaver start time.
I am at Settings->Theme->Screensaver start time. It gives you two choices: 1 minute or 2 minutes. That is not disabling the screensaver. Not at all. It's torturous to give us the ability to have our photos on our TV when no matter what it's going to go to screensaver after 1 or 2 minutes.
What Roku model do you have? On my 3941, I have Disable, 1 minute, 5 minutes, 10 and 30 minutes.