In iFile or SSH, from root follow this path:
Applications>preferences.app>general.plist
Halfway down you'll see this:
setScreenLock:specifier:
validTitles
1_MINUTES
2_MINUTES
3_MINUTES
4_MINUTES
5_MINUTES
NEVER
validValues
60
120
180
240
300
-1
Change the 60 integer to 30 for 30 seconds or to 15 for 15 seconds etc.
You can also change the string from 1_MINUTE to 30_SECONDS
Reboot or respring when done.
REMEMBER: to backup the plist before editing it.