Actually, about 7.5%, but you have to add in the employer's share, so the real tax is over 15%.
Near as I can make out, the mean average contribution in 2002 was about $3,300 (136m employed paying $440b). The average person retiring today would certainly have paid more in 2002, perhaps $8,000, but he would have started the 45 years at more like $1,000.
Start at $1000
Increase contribution 5% each year, up to $8000 in 45th year
Accumulated contribution earns 5% per year
Ending accumulation = $380k
If this were paid out over 20 years, and the balance continued to collect 5%, then the annual payout s/b about $30k. The actual maximum is about $20k. You wouldn't have to lower my starting and ending points ($1,000 and $8,000) much, though, to bring this into line.