Number of hours exercised must equal the number of extra years lived to make back the time investment in the exercise. To be a worthwhile investment, amount of time spent exercising must be less than the number of extra years lived, thus you are gaining time.

So assuming every day you exercise 1 hour, and that this would extend life expectancy by 3 years, the extension of 3 years would equate to 26,280 hours of life extended. Therefore if you exercise for an hour a day from birth, at the age of 82, your investment would balance out, and any further exercise investments would not pay off. This is assuming the lifespan extended by is static.