Go to... Settings > Display > Screen timeout... select, 15, 30 sec's, 1, 2, 5, 10 min's
I had this same problem. I kept trying to change the screen timeout from the display settings but it would always say 10 mins no matter what I selected and turn off after 20 seconds. See if you have any apps that may affect the screen timeout. I had an app that I used call Kin screen to keep my screen on whenever I needed it to just stay on for an indefinite period of time. I didn't realize that it had a setting in there also to turn the screen off and it was set to 20 seconds. Made the change in the app and it fixed the problem.