If you are not vaping yet - this covers the two most popular small models
http://www.e-cigarette-forum.com/fo...st-electronic-cigarette-first-time-vaper.html
100mah of battery charge is about 1 hour of moderate vaping with standard resistance atomizer/cartomizer (which is all a slim ecig can use - they can't use the hotter low resistance stuff)
For slim ecigs - manual batteries are generally a smarter choice. Manuals are harder (some impossible) to damage or kill with juice leaks, won't turn on accidentally in wind or vibration, make it easier to get a good drag and it. We refill everything with eliquid so sometimes leaks occurs.
Heavy smokers, hot hard draggers, people who away from AC power for long periods are best off skipping the slim scigs and going straight to fat battery models and mods.