It slows when you sleep or are in a very calm state. Expanding this concept, heartbeats are also individual to species. Smaller creatures have faster heart beats than larger ones.
So you would expect all beings to have a different number of heartbeats in their lifetime right?
Wrong. Living creatures, despite their differences, have a similar number of heartbeats during their lifetime. This number is around 1.5 billion.
Large animals live longer but they also have slower heart beats. Smaller animals live shorter lives and have more frequent heart beats. Hence, most creatures have a similar number of heart beats.
If you think about time as measured by heartbeats, all creatures experience the same length of life - a lifespan of 1.5 billion heartbeats.