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!

This one works too:
http://www.filife.com/calculator/the-earlier-you-start-saving-the-happier-youll-be

In addition to learning what percentage you should be saving, serious consideration should be given to the asset allocation in your 401k. You have a very long time horizon, which suggests that at least 80% of your investment assets should be in equities, both domestic and international. The remainder could be spread among alternatives, such as real estate, commodities, currencies, and short-term fixed income securities. This, of course, will depend upon the options available to you through your 401k plan.

And this one is for after you retire:
http://www.filife.com/calculator/how-much-can-i-spend-when-i-retire

But if you do it right you won't need to budget!

Dude, here's the tool:
http://www.filife.com/calculator/how-much-do-i-earn-by-saving-through-a-401k