> The maximum cycle length is 2256 ≈ 1.16×10^77 iterations. If you can evaluate 10^12 hashes per second, then working your way through all possible hashes would take you about 10^65 seconds (about one quindecillion times the age of the earth). Even if you're fortunate enough find a loop in a tiny fraction of that time, you're still liable to be waiting for trillions of years.
https://stackoverflow.com/a/43636715
Edit: fixed missing exponent notation