duration
Returns the duration (milliseconds).
See also
https
://aws.amazon.com/ko/blogs/architecture/exponential-backoff-and-jitter/
implement "Decorrelated Jitter" temp = min(cap, base * 2 ** attempt) sleep = temp / 2 + random_between(0, temp / 2) sleep = min(cap, random_between(base, sleep * 3))