First of all, never buy another pack of cigarettes once you've quit. If need be, buy a disposable e-cig like NJoy or Blu.
Lithium ion battery life expectancy is measure in charge cycles. Basically about 250 - 300 charge cycles is the life expectancy.
However, how well you treat your battery determines the life expectancy. Try not to completely drain your battery before you recharge it. Full drains are hard for the battery to recover from. Charge the battery with the charger it came with. Use a USB/wall adaptor for the charger, not a USB computer port or an car's USB port.
Try not to leave a fully charged battery on the charger, remove it when charged. A charger will stop a full charge, but will continue to give a trickle charge to keep it topped off. Batteries don't like this, and this will decrease the overall life expectancy.
Keep the top 510/ego connector clean from e-liquid. Dried juice corrodes the metal.
This. Baditude covered all the necessary points about battery life. Note that a charge cycle is from 0% to 100% (for Li-Ion batteries); as such, if you plug your battery into a charger at 50% and charge it to 100%, you have "used" half a cycle.
So, if you keep your battery from discharging fully, you will likely enjoy a longer battery life - however, occasionally (or even all the time) discharging your battery will not actually hurt it. You are more likely to hurt it by discharging it at a rate that is too high...
Assume V = 4.2 volts, R = 1.8 ohm
P = (4.2V)^2 / 1.8 ohm
P = 9.8 Watts, for a current (I) of 2.33 A.
Not much, considering most batteries out there (especially variable voltage) can certainly output way more than 2.5 amps. However, you must consider the internal resistance whose voltage drop becomes significant at higher currents. (try draining 2 amps out of a AA battery)
Even with sub-ohm internal resistance, you are likely to dissipate significant power
inside the battery. Such is the nature of electronics; therefore, turning down that variable voltage battery at the compromise of vapor production will increase your apparent lifetime. So will using a slower charger (less current), at the expense of waiting forever for a charge to complete!
It's a compromise - and frankly, we haven't even looked at the effects of temperature (heat, actually). Should we care? I think not - batteries are definitely cheaper than analog cigarettes!

The rule of thumb, though, is about 300 cycles - and it's mostly accurate. At that rate, provided you can squeeze a day's worth out of a single battery - it should last you
at least a year (probably more).
EDIT -- as diamondlou below me said - don't over torque components. I realize after posting this that your battery is probably likelier to fail due to mechanical reasons and **not** from deterioration of its electrical characteristics.