In 2012, you get one credit for each $1130 in earnings you have up to a maximum of four credits per year ($4,520 for the maximum four credits). The amount of earnings needed to obtain one credit increases each year.
Most people need 40 credits (which takes 10 years to accumulate) to be eligible for benefits, although disability or survivor benefits may require fewer credits.
Although you may obtain many more credits than you need to be eligible for Social Security benefits during your working lifetime, the extra credits do not increase your benefit. What does increase your benefits is the amount of income that you earned and made contributions on.