How much does a dell xps15 laptops gives battery life with a 6 cell and 9 cell lithium batteries?

I have asked this question before. But does not satisfied me. I need to know the above laptops battery life. When i checked the dell website, according to them the xps 15 of 6cell battery gives 5hours and a 9cell battery gives 8hours. Is it is really gives such a battery life or is the company is giving it as fake for marketing?|||the numbers given are just a guess it depends what you are doing on the computer how long they will last. if you are burning a dvd and on the internet no time at all...