Manufacturer's estimated battery run times are heavily skewed in their favour. Test conditions to get these results are usually ridiculously impractical. Test systems are often run at very low brightness levels with things like the wireless adaptor, bluetooth, etc. turned off in order to squeeze out every last second of battery life to make the model's spec sheet look the best.
That being said, typically, 3-5 hours is fine when the system is off and charging with a dead battery. Most systems have the battery already charged on delivery so it may take less time. If you want to be sure that the battery is completely charged, plug it in and leave it overnight.Accu Dell Inspiron 1750 http://www.accukorting.com/dell-inspiron-1750.html