I'm sorry to hear that you're having some battery length issues.
It can depend on how many apps you're running at the same time, as well as the screen brightness and if you have GPS, 4G and Network (LAN) options enabled. If you're not in a 4G area, you might as well turn that feature off. Same with GPS if you're not driving or locating something.
There are also some apps that are designed to limit or turn off running apps that you might look into downloading for a quick disabling of programs when you aren't using them.
I had purchased Galaxy 5 mobile phone recently. It takes very long to charge it to 100%. Also, charge comes down very quickly, by around 8 hours when kept UNUSED!! I cant understand what's happening. How to solve this. I am new to samsung as well as android!