Then you either have a fault with your phone that you should get repaired, a rogue app that you should uninstall, or a problem with your settings that you should fix.
The Lumia 720 has a large battery and should last at least a couple of days with moderate use.
I have been always set to 3G, what is the point of having it on your device and not using it? not unless your saving battery charge. If data is off or stangby it should choose the correct connection to save battery. This is not andriod.
I'm always-on - BT, NFC, 3G, only turning off when saving battery. I usually make it through the day on weekends and have several % left, during work week it sits on my "horizontal" wireless charging stand all day