Battery life on most phones is greatly reduced by using your data plan.
On any of the phones I have had I always have the data plan turned off. I don't need it on unless I am sitting and browsing the internet and there is always WiFi everywhere anyways now.
If you can, try to live without your data plan. The data chip draws 3-5x more power than the WiFi chip. I had a phone that was only lasting 6 hours even just on standby, turned off the data plan and it would go 2-3 days on standby.
If you are using it for "on-screen" time, keep in mind that the display draws a lot of power. If you want to make it last longer, turn off adaptive brightness and keep it at a lower setting. I keep my Nexus 6 just below halfway on the brightness setting, thing lasts 3 days without a charge with normal use.