Multiple definitions of the Sterling ratio exist. The original definition was most likely suggested by Deane Sterling Jones (a company no longer in existence):
Typically the time period is over 3 years and only the largest individual drawdowns are included (e.g. 3 or 5).
Bacon, Carl, Practical portfolio performance measurement and attribution 2nd edition, Wiley 2008, ISBN 978-0-470-05928-9