The lifespan of lithium polymer batteries is measured in completecharging cycles rather than months or years.
Well maintained LiPo batteries last from 300-500 complete charge cycles. As you don't usually allow complete discharge you are only performing partial charges -- so the total number of charges will therefore be greater. The battery won't be dead at this point but around 80% of original capacity with markedly reduced performance and reliability.
The Spark has only been released for 18+ months so you would have had to fly it about every day to "wear out" a battery.
They auto discharge starting at about 6 days of idle storage - as long as you don't repeatedly run them too far down and don't expose to extreme temps you'll have no issues.