it really depends how heavily your using your phone and what for
I have 2 Motorola BH5X Standard 1500mAh Batteries I keep on me for backup at all times, which, ever since my sister-in-laws wedding and a trip to disneyland has become the standard for me.
Turns out taking a butt load of pictures and helping my wife manage the people at a wedding required a total of 3 batteries for my phone
Meanwhile, taking a lots of pix at Disneyland with no readily available outlets for my external charger also made 3 batteries the perfect number, especially after recording those late night light shows.
On a regular day though I can stretch out a single battery if I have an outlet for charging nearby or 2 batteries without a charger.
and yeah, I don't restrict data either so that does a number on my charge.
Samsung Galaxy Note II