Many factors affect the amount of time that a portable computer battery can deliver power before it must be recharged. These factors include the configuration (processor, memory, etc.), types of applications being run and display brightness.
Dell uses industry standard benchmarks for battery run time claims. For more information on battery run times and benchmarks, the white paper, "Understanding Battery Life in Portable Computers" is available at:
http://www1.us.dell.com/content/topics/global.aspx/vectors/en/2005_batterylife?c=us&cs=555&l=en&s=biz