1. Swipe from right edge to bring up charms menu
2. tap search, type "power"
3. tap on "settings"
4. tap "edit power plan"
5. change when to dim the display, put computer to sleep, and adjust plan brightness.
6. Click "Save changes" button.
Hope that helps!
You can also hit "windows key + X" to bring up a menu with a bunch of options including the power options.
keep in mind that this is ignored if you connect your surface to your company's exchange service using activesync. the workaround I found for this is either to convince your company to increase their default(good luck with that one) or turn on presentation mode on your surface which means it never turns off the screen if that is better for you...