Part of the appeal of Chromebooks lies in their lightweight minimalism, but that does mean you miss out on some advanced OS features — such as being able to set a display time-out interval. If you want to stop your Chrome OS screen from dimming then there's an extension that can help.
The extension in question is Keep Awake and with nearly 150,000 users at the time of writing it's obviously a plug-in that a lot of people rely on. Once installed you can click to cycle through the modes: keep the screen on (sun), stop the system from sleeping (sunset) and leave the default settings unchanged (moon).
Use the sun mode to keep your display on at all times; use the sunset one if you don't mind the screen dimming but do want to keep network connections active. At the moment these switches are only available to developers but let's hope they make it into the stable versions of Chrome OS in the near future.