borrowed time
An uncertain length of time that may end soon or suddenly, bringing any activity, situation, or fortunes associated with it to an end as well. Usually refers to the final period of one's life, in the form "living on borrowed time." In the scope of the planet's history, human existence is a tiny blip and unlikely to last forever—just borrowed time, really. Accidents and disease can strike so unexpectedly that it feels like we're all living on borrowed time.