Dell India — latitude e5400 battery backup

I got a new 4 cell battery for my dell latitude e5400 (Original battery from dell). While purchasing the battery, it was written that i would get backup for 2 to 2.5 hrs but i am not getting more than 2 hrs of backup. While performing my daily activities (Mainly over internet), i get a maximum backup for 1hr 45 minutes or so. Please let me know the general back up time.