I use an HP Dv2988 Special Edition laptop with a 12 cell battery and the battery with power saving mode and reduced Brightness should last yu about 4-51/2 Hours depending on what program yu are actually running, it comes with a 4GB RAM so i run all my programs in power saver mode.
I use a HP Pavilion DV6000 PC with a 12 Cell Battery and it runs for 4hours in Power Saver Mode with reduced brightness when reading and listening to music and just over 3Hours when watching movies.Kinda Unimpressive knowing i had to cough out 22,000 for it.
i don't really the cell of my DELL LATITUDE D531, but using balanced mode, and on microsoft packages, (word excel) 8hours, browsing 4.5 hrs, music, 6hrs, films, 5hrs.guess DELL is a machine when it comes to batteries.