For the highest "return" on your SS investment, you should work long enough to have earned $342,720 in total SS-eligible earnings (in todays dollars). Up to this point, you get the .9 multiplier.
And the "second bend", from the .32 multiplier to .15, is at $2,065,400 SS-eligible earnings.
That is roughly $58,000 yearly gross income from work over 35 years.
Beyond that point it becomes a rather pricey insurance when compared with other annuities (unless one counts the employer contribution - then it doesn't look so great, but not too shabby either given spousal benefits and disability)
Peter