“I believe every human has a finite number of heartbeats. I don’t intend to waste any of mine running around doing exercises.”
– Neil Armstrong (5 August 1930 – 25 August 2012) (1)
There are 86,400 seconds/day, and 31.536 million seconds/year (365 days).
The normal resting adult human heart rate ranges from 60 to 100 beats-per-minute (bpm). Slow heartbeat rates of about 40-50 bpm during sleep are common and considered normal. Medically, heart rates of 50 to 60 bpm in apparently healthy people are taken as a good sign needing no further attention, while heart rates above 80 bpm may be due to some otherwise undetected unhealthy condition, if not caused by stimulants like caffeine, or bursts of exercise. The maximum heart rate a person can safely experience during bursts of strenuous activity decreases with age, being about 180-200 bpm for people in their 20s, 175-190 bpm for people in their 30s, 170-185 bpm for people in their 40s, 165-175 bpm for people in their 50s, 155-170 bpm for people in their 60s, and 145-160 bpm for people in their 70s. A human lifespan that is not prematurely interrupted may experience up to 3.5 billion heartbeats, or even more. (2)
Let us define a characteristic average heart rate, which we shall call the Armstrong Heart Rate (AHR) in honor of Neil Armstrong: test pilot, aeronautical engineer, university professor, and the astronaut who was the first human to step onto the surface of the Moon. Assume as typical an average heart rate of 66+2/3 bpm during three quarters of every day (18 hours), which includes periods of “calm” and periods of “activity” and “stress.” We assume that sleep occupies one quarter of every day (6 hours) with an average heart rate of 40 bpm. The daily average with these assumptions is
AHR = [3/4 x (66+2/3)] + (1/4 x 40) = 50 + 10 = 60 bpm = 1 bps (beats per second).
A human with a heart rate equal to 1 bps will experience 31.536 million heartbeats per year. Given this average heart rate, the total number of heartbeats over periods of time would be as follows.
The United States is listed 38th and ranked 34th among nations as regards average life expectancy. The overall life expectancy in the United States is 79 years. The U.S. is ranked 37th for male life expectancy, which averages 76 years, and it is ranked 36th for female life expectancy, which averages 81 years. (3)
By our AHR model of average heart rate, the average US male lifespan includes 2.396736 billion heartbeats, and the average US female lifespan includes 2.554416 billion heartbeats. The overall average (79 years) is 2.4913344 billion heartbeats.
So, the average US lifetime is one of about 2.5 billion heartbeats, assuming the typical heart rate is the AHR, which we defined as 1 bps.
Of course, heart rate can and will vary over the course of a lifetime, and human variability is wide, so in reality heart rates both above and below the AHR model will occur in the population. The AHR model helps us visualize the order of magnitude of total heartbeats experienced in a human lifetime.
The heartbeats per lifetime for a wide variety of non-human mammals ranges between 0.53-1.5 billion heartbeats; and is 2.17 billion for chickens that live 15 years, and 2.21 billion for humans that live 70 years. (4)
Since many animal species experience lifespans of about 1 billion heartbeats, we can think of them as “dying in our 30s.”
We can describe five stages of human life, based on the summation of heartbeats, as follows:
1 billion heartbeats to develop and grow into seasoned adults in three decades (to 31.71 years),
1 billion more heartbeats to experience three decades of productive adult life (to 63.42 years, 2 billion heartbeats),
1/2 billion more heartbeats over the course of 1.5 decades of retirement and denouement (to 79 years, 2.5 billion heartbeats),
a possible bonus of another 1/2 billion heartbeats and 1.5 decades of advanced old age (to 95.13 years, 3 billion heartbeats),
and a very few may experience another 1/2 billion heartbeats to live another 1.5 decades of extreme old age (to 111 years, 3.5 billion heartbeats).
For most of us who manage to avoid the fatal hazards of bad luck and disease, we can expect to experience lifespans of between 2 to 3 billion heartbeats, and most likely about 2.5 billion heartbeats.
The wise thing to do with your heartbeats is to spend the life they sustain on what you enjoy doing.
The only moral constraint (or aspiration) I would put on that enjoyment is: be kind.
 Neil Armstrong, https://en.wikipedia.org/wiki/Neil_Armstrong
 Heart rate, https://en.wikipedia.org/wiki/Heart_rate
 List of countries by life expectancy, https://en.wikipedia.org/wiki/List_of_countries_by_life_expectancy
 Animal longevity and scale, http://www.sjsu.edu/faculty/watkins/longevity.htm