H = Hashrate (hashes / second)
D = Difficulty (Reference for values below)
B = Reward per Block (Reference for value below)
N = Number of days per month (default = 30)
S = Number of seconds per day (S = 60 * 60 * 24 = 86400)

H = 21,990,232,555,520 h/s (~= 20TH/s)
D = 47,427,554,950.648
B = 25
N = 30
S = 86400

$ python -c "print (30*21990232555520*25*86400)/(47427554950.648*2**32)"
$ 6.99542703277

time = difficulty * 2**32 / hashrate