Not quite sure what you are asking here. Are you asking about the hardware screen? or the function screen?
Your hardware screen (monitor and display adapter) is controlled by hardware and operating system. The MineOS overlay / WebUI have no control over this. If you are using the turnkey distro of MineOS it is pretty much a rebanded debian. The following link should give you more info on adjusting screen timeout: http://askubuntu.com/questions/138918/how-do-i-disable-the-blank-console-screensaver-on-ubuntu-server
(The links discuss disabling, but read the entire thread, near the botom you get the settings for adjusting by number of minutes)…
As for the function screen:
I really cant see any use for a timeout here, since this would effectively kill your minecraft server, with a lot of unwanted effects (like loosing work since the minecraft server do not get a chance to shut down gracefully).