If other solutions (using control panel, using the 'xset' command with various options, etc.) do not solve this for you, you might try this:
sudo apt-get remove phoneui-apps phoneuid phonefsod
Explanation:
I tried numerous combinations of xset commands, and every GUI I could find that claimed to relate to screen blanking, but nothing worked - most of the time, the screen would blank after ~33 seconds of inactivity.
It occurred to me that perhaps some wayward daemon was doing this for some reason, so I looked at system logs, and found this in /var/log/auth.log:
Jan 3 15:48:01 my-host-name dbus[886]: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.8" (uid=0 pid=1115 comm="/usr/sbin/phonefsod ") interface="org.freesmartphone.Device.Display" member="SetBrightness" error name="(unset)" requested_reply="0" destination="org.freesmartphone.odeviced" (uid=0 pid=959 comm="/usr/sbin/fsodeviced ")
Jan 3 15:48:14 my-host-name dbus[886]: message repeated 2 times: [ [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.8" (uid=0 pid=1115 comm="/usr/sbin/phonefsod ") interface="org.freesmartphone.Device.Display" member="SetBrightness" error name="(unset)" requested_reply="0" destination="org.freesmartphone.odeviced" (uid=0 pid=959 comm="/usr/sbin/fsodeviced ")]
Jan 3 15:48:23 my-host-name dbus[886]: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.8" (uid=0 pid=1115 comm="/usr/sbin/phonefsod ") interface="org.freesmartphone.Device.Display" member="SetBrightness" error name="(unset)" requested_reply="0" destination="org.freesmartphone.odeviced" (uid=0 pid=959 comm="/usr/sbin/fsodeviced ")
Jan 3 15:48:31 my-host-name dbus[886]: message repeated 2 times: [ [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.8" (uid=0 pid=1115 comm="/usr/sbin/phonefsod ") interface="org.freesmartphone.Device.Display" member="SetBrightness" error name="(unset)" requested_reply="0" destination="org.freesmartphone.odeviced" (uid=0 pid=959 comm="/usr/sbin/fsodeviced ")]
Jan 3 15:48:41 my-host-name dbus[886]: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.8" (uid=0 pid=1115 comm="/usr/sbin/phonefsod ") interface="org.freesmartphone.Device.Display" member="SetBrightness" error name="(unset)" requested_reply="0" destination="org.freesmartphone.odeviced" (uid=0 pid=959 comm="/usr/sbin/fsodeviced ")
Jan 3 15:49:01 my-host-name dbus[886]: [system] Rejected send message, 1 matched rules; type="method_call", sender=":1.8" (uid=0 pid=1115 comm="/usr/sbin/phonefsod ") interface="org.freesmartphone.Device.Display" member="SetBrightness" error name="(unset)" requested_reply="0" destination="org.freesmartphone.odeviced" (uid=0 pid=959 comm="/usr/sbin/fsodeviced ")
The references to "Device.Display" and "SetBrightness" seemed suspiciously relevant. I don't specifically recall installing these packages, but I was trying to extract data from a semi-functional mobile phone a while back, so I probably did it then. In any case, I knew I didn't need these packages now, so I removed them (see 'apt-get remove' command, above), sat back, and waited for the screen to blank... and waited some more... and it didn't. :-D
Again, I suggest trying the other possible solutions (system settings GUI, xset commands, etc.) first, simply because they apparently solve the problem for some people, and they seem more likely to work. OTOH, unless you happen to be using these packages, removing them is harmless (and if they're not installed, apt-get will just say so and you haven't hurt anything). Hopefully, this will help someone...