At what time do I consider myself postmenopausal?

No periods. When you have not had periods for the past 6 months-12 months and are of age (can happen early, however) and your ovaries no longer work, you are post menopausal. When your periods change at this time and slow down and are infrequent, you are peri-menopausal. Blood tests can prove this, but it is usually a clinical diagnosis. Discuss with your pcp/gyn on your yearly visit unless you have problems.
Postmenopausal. Means that a woman has not had a period for a year. Then they are considered postmenopausal.
Forty plus. Post menopausal is the term used when women stop menstruating and usually occurs in the forty plus.