I don't have a Latitude laptop but i do own an e1405 and e1505 which are the 14.1" and 15.4" inspiron laptop models.  Both have 9 cell batteries.  If I have all of the settings turned on to Max Battery life i can get about 6 hours on the 15.4" and around 7.5-8 hours on the 14.1".   I have never used a 6 cell battery but according to dells website you should get around 3-4 hours on the 15.4" and it dosent say anything about 6 cell battery life on the 14.1"  However these numbers might be a little different for you considering its a different laptop line but they should give you a general idea.