If your phone has been switched on (even if you've not made or received any calls) it's still used quite a lot of power. The cellular network relies on your phone periodically sending out a "I am here" message to the local masts. (You've probably heard those signals if you've ever left your phone near to a radio or to your PC's speakers). After sending out that 'location' signal, your phone waits for a '"Thank you - message received" reply from a local transmitter. If it doesn't get one (because you're in a fringe reception area) it re-sends the location signal, at increased power. If it still doesn't get a reply, it tries again with yet more power.
So if you happen to spend quite a bit of time in a poor reception area (or somewhere like Southwold promenade where there's no signal at all for Vodafone) your phone will constantly be sending out high-power 'location' signals, while it tries to contact a local mast.
Consequently your mobile phone's battery can be drained quicker in some locations than in others. So there's no fixed rule about how long the battery should retain a charge. However I'd expect a new battery to typically last a week or so before going flat (assuming that the phone is switched on but not used for calls), whereas an older battery might need charging after two or three days.
Chris