Many factors affect the amount of time that a laptop battery can deliver power before it must be recharged. Personal usage (your LCD brightness setting) has a large effect on battery run time and will vary greatly between different users and systems. Other factors include the system configuration (battery size, processor type, memory, etc.), and the types of applications being run (playing DVDs or games).
Dell uses industry standard benchmarks for battery run time claims
Actual battery life will vary based on configuration, operating conditions and other factors. Maximum battery capacity decreases with time and use.