Actually ....
On average I think (today) it takes 11-12 years on the SS 'dole' to recoup the 'actual' (without interest) money an individual pays into the system if they retire at 65.
And I think the 'average' will (maybe in the next 5 years or so) be rising to 13-14 years to get 'your' money back.
I've had earned income above the SS maximum almost since I first started working. That is the WORST situation in terms of "efficiency" of payback, since about half of my SS income is above the so-called second "bend" (where each additional average dollar earned per month adds only 15 cents per month in SS benefit). Yet, when I compute how much I've paid into SS over my lifetime (and extrapolate for the remainder of my career) and I compare that amount to the the estimate of how much I'll get paid back in SS benefits each month, I think it will take me only about EIGHT years to get paid back - that's the WORST case.
For those with lower incomes, the situation can only be better. For example, those with incomes below the first "bend" (which is about $760 in 2009) get paid 90% of each average monthly dollar earned. Thus, someone with an average monthly SS wage of 700/month over the course of their career will get paid back 630/month in SS income.
And don't forget that to compute the average SS wage of an individual every past dollar earned is "indexed" to current dollars based on the "National average wage index" for the year in which the dollar was earned. For example, a dollar earned in 1970 gets indexed to $6.68 2009 dollars for the purpose of computing a person's benefit in 2009 (because the average wage was $6,186.24 in 1970 and is $41,334.97 in 2009 - 6.68 times as much).
Putting it all together, the example "first bend" person above will have paid only about 1/13th as much as I have in SS taxes, but will receive a monthly SS benefit greater than 1/3rd my benefit. Thus, the payback period for such an individual will be MUCH faster - less than TWO years.
Edit: Edited to improve clarity.