Well, you would need a script that polls for button presses. Then you would add a timer to that script. The timer would need to be set to run your screen saver every 900,000 ticks (1000 milliseconds X 60 seconds X 15 minutes). Then you would need to reset that timer every time that there was a button press. I bet if you google it, you can probably find someone that has already made something that you can use. I would try googling something like 'unix command line screensaver' or 'python screen saver'.