Most of the time Keep screen on is only an option for when the device is plugged in, and not a setting in the display timeout. Since we don't know what device you have I will give the directions which should approximate to your device; Go to settings>Display & gesture>Advanced and there you should find Screen timeout. I imagine that you found the other under your power settings, which is a logical place to look as most want to control their screen on time to control power consumption but what you are looking for should be under Display, Display & gesture to be more exact. Again not knowing what device you have, I have a search option at the top of my settings menu where I can begin to type and it will predict what I'm looking for. If you have this option you could try typing "timeout" and see if that gets you to the setting any easier.