I have my phone drop into 'phone only' mode at 20% battery. From that point, the amount of battery use is negligible. I believe all the hype about 'standby time' now. I think it could last a week at least like that from a full charge.
The fact is it is good at conserving battery until you use the other features. Then it drains like crazy in comparison.
So to get great battery life:
Turn off WiFi
Turn off mobile data
Turn off auto-sync
Turn screen to 1% (I find 0% turns the screen off!)
Turn off GPS
Turn off bluetooth
Kill all unused apps & services
Turn the screen off.
Or, you could buy a 'dumb' phone, of course! That is effectively what you are doing by this.
Living in the tropics and loving it