How long does it take on average for a human body accelerate to 78 mph in freefall?

Convert 78 mph to meters per second:

78 mph=35 meters/second

The acceleration of earths gravity is 9.8 meters per second squared. This is constant. Therefore, using basic calculus, the velocity of a falling object is equal to 9.8 meters per second times the time it has fallen.

9.8*T=35

T=3.6

Thus, it takes roughly three and a half seconds for the average human body to accelerate to 78 miles per hour (assuming wind drag is inconsequential).