Benji,
As some of the calculators above indicate, there's not necessarily a one-size-fits-all answer to this. The more you save, the more you'll have when you retire, but how much you need depends on what your goals are.

All that being said, the typical "rule of thumb" is that people in their 20s should aim to save about 10%-15% of their gross income. That will allow you to build a steady retirement savings over time, and will also get you into the healthy habit of spending less than you earn - a major key to long-term financial success. If you can afford to save more, that can just help to get you to retirement faster, and/or allow you to live a more expensive lifestyle in the future.

I hope that helps a little!