The larger the battery's mah (milliampere hours)rating, the longer a battery will last between charges. You also have to pay attention to the resistance of atomizers or cartomizers you're using. That is, the resistance of the atty/carto should be compatible with the size battery you're using.
Generally the lower the resistance (ohms) rating on an atty/carto the quicker it'll drain a battery. The advantage of lower resistance is that more vapor is produced, but if the battery you're using won't handle the load, the battery, atomizer or cartomizer can be damaged/burned.
____________________________________________________________________________
Here's some info from a compatibility chart I found in the Madvapes website knowledge base:
510, 901,808 and other China batteries: 2.5 ohm is the standard but they will work as low as 2.0 ohms. Higher resistance produces less vapor. Below 2.0 ohms, the battery life is signifigantly reduced and the batteries may be damaged
eGo: 2.0 - 2.5 ohms. Same as above.
3.7 Volt mods, roughstacks, box mods, e-Power: 1.7 to 3.2 ohms Above 3.2 ohms, vapor production will be reduced. Lower than 1.7 ohms, battery may think it's a short and not work.
_____________________________________________________________________________
From personal experience, two Riva 750 mah batteries (these are Ego style) will get me through an average day of vaping with 2.5 ohm cartomizers - a little less than a day using 2.0 ohm cartomizers. Of course, it can't be said enough that what is an average day for one may not be for another....
Hope this helps