log(0.5) / log(999. / 1000)

692 / 60. 

import timeit
from random import random

def time_me(n = 10000):
    z = 0.0
    for i in xrange(n):
        z += random() - .5
    return z

timeit.repeat(time_me, repeat = 3, number=10)

import psutil
get_time = lambda : psutil.Process().cpu_times().user
t0 = get_time()
loops = 100
for x in range(loops):
    _ = time_me()
print (get_time() - t0) / loops