Welcome to iMore! It could a number of issues and battery life can vary based on your usage. How long can you go between charges? What kinds of things are you doing on your iPhone?
Start by going to Settings > Battery, and looking at your usage. Do you have any apps using up a bunch of battery that you don't really use that often? If you tap the little clock icon under your battery usage you can see your usage time, both on screen and in the background, for your apps. This should give you a pretty good indication if you have a rogue app taking up excessive juice.