Keep in mind that using a camera that captures GPS data to modify the EXIF data in the picture and then sending ~1Mb - 4Mb file sizes over a cellular network requires some battery. I'm not a snapchat user but never wanted to be one because running the app is just another layer of battery usage that is already getting drained pretty hard.
It's about like this
Opening the app - uses battery until the app closes
The app processing your chats
Gathering facebook or user info for changes in friends
Gathering updated photos
The app opening Camera to take a picture uses battery until camera closes
Camera takes a picture
GPS data is gathered for the picture
picture is saved
OR.... Picture is selected
Photos is opened and closed when the picture is selected
Picture is closed
the app dismisses the camera and allows you to add text
the app reprocesses the image and adds the text.
the app then sends the data out for approximately 5 seconds (on a decent network)
(loop for as many times as you send photos)
Close the app (run shutdown tasks like storing data to the disk for standby)
That is a lot whereas you'd only need to do the camera section and use your messages to send a message otherwise. There is no extra layer that would cause your phone to die even faster. If there are ads, then every time an ad pops whether it's full screen or just an ad bar, that data still needed to be downloaded to your device. This is also assuming that Snapchat doesn't use data collection to send back data about your usage in the application back to the developer or a 3rd party advertising agency.
I have nothing against snapchat and I'm not trying to deter anyone from using it, I'm just saying there is a lot going on, so if battery life is a problem for you and snapchat is to blame, let the selfies wait until you're on wi-fi or 5 bars on a fast network.