Freefall to sea level

How long would it take to fall from different levels of the atmosphere to sea level at freefall speed?  (80 km, 600 km, and 800 km) 

you will actually never reach terminal velocity. as time goes to infinity, the velocity approached terminal velocity. but usually it's like an asymptote.

v(t) = sqrt(mg/b) * tanh[t / sqrt(m*g/b)]

time at terminal velocity would be:

v_terminnal = sqrt(m*g/b) = sqrt(m*g/b) * tanh[t / sqrt(m*g/b)]

=> 1 = tanh(t / sqrt(m*g/b)

=> atanh(1) = t / sqrt(m*g/b)

=> t = atanh(1) * sqrt(m*g/b)

inverse hyperbolic tan (atanh) of 1 is infinity. so you never really reach terminal velocity, just gets really really close to it, so the difference doesn't really matter.