Hi Lobengula, let me explain: A spark of life actually lasts approximately 52 game years, or one real year. However, if your character suffers trauma, such as being incapacitated in combat or dying, the character will lose some of that spark of life. So while a spark of life generally lasts about a year of real-time, it can go out earlier depending on how you play and what happens to the character in that time.
Hope that helps! :)