No, you can purchase sparks of life as stand alone things. They are more or less what Soulbound Studios uses for their "Subscription"

Pledging in many cases will provide you will sparks of life, as well as other benefits, but you can always get sparks by themselves. The current rate being $25 getting you an average of 52 weeks of play.

As well as the above information, it should also be noted that a single spark of life can last anywhere from 3 to 14 months in out-of-game time. This range is dependent on how dangerous your in-game lifestyle is (prone to go into dangerous dungeons versus a craftsman in a city) and how much someone wants you dead (a king is more likely to get assassinated by a rival than a farmer).

I believe that you would enjoy reading DJ #16, which goes over a lot of information on life and sparks.

http://chroniclesofelyria.com/blog/1772/DJ-16-The-Weight-and-Measure-of-a-Lifetime

It should be said that considering the cooldown on death penalties you would have to be at least a bit famous (increased penalty multiplier) and die pretty much daily to manage to shorten your spark down to only 3 months.

Also, it's worth clarifying, as it seems the OP is confused about this: your character is mortal, once your spark of life runs out you will have to spark into a new character. You will be able to spark into a descendant or a relative though, which means you're able to inherit the properties and items left behind by your previous character. In addition the skill ramps of the soul will also help you recover you skills faster.

In other words, it doesn't matter if you run out of sparks, the result is the same whether you have spare sparks on your account or buy a new spark after your character perishes. You simply just need a spark to allow one of your souls to manifest into a new character.