How to increase passcode lock on iphone 3gs

How to increase the passcode lock time from 15 minutes to 1 or 4 hours:
a. Jailbreak phone.
b. SSH into the phone as [ root ]
c. Edit the following file [ "/var/Managed Preferences/mobile/com.apple.springboard.plist" ] Just double click on the file to edit.
d. Make the following change, setting of the key "maxGracePeriod" integer from 15 to 480. Also change the "maxInactivity" integer also from 15 to 480. Even though 480 is 4 hours, when you go back to preferences, your passcode lock will have (additionally) the 1 hour and 4 hours options.
<key>PolicyInformation</key>
<dict>
<key>SBAutoLockTime</key>
<integer>900</integer>
<key>SBDeviceLockGracePeriod</key>
<integer>900</integer>
<key>forcePIN</key>
<string>0</string>
<key>manualFetchingWhenRoaming</key>
<string>0</string>
<key>maxGracePeriod</key>
<integer>480</integer>
<key>maxInactivity</key>
<integer>480</integer>
</dict>
Make sure and "save" the changes. and exit from the SSH program.
e. Now go into passcode lock "Require Passcode" to 1 or 4 hours.