I too have a Lenovo x220. I use Jupiter [0] to manually send the computer to "power saving" mode. With that and the extended 9-cell battery, the screen only slightly dimmed, and wifi at full blast, I get around 7 hours of battery doing browsing and coding. I uninstalled Flash, and that gave me back around 2 hours of battery.

[0] YMMV: http://askubuntu.com/questions/98972/does-jupiter-really-imp...