How will you spend your retirement? Will it be relaxing, traveling, enjoying the good life? Or will you be flat broke, working until you drop dead? Plan ahead - figure out how much money you need to save for retirement! Assuming that you plan to live forever, you should plan on having at least twelve year's salary saved up when you retire.
That's actually not as hard as it might seem.
Assuming you save 20% of your income each year and work for 40 years, that's already 8 year's salary - you're two thirds of the way there! Of course, it's not as simple as that; for one thing, your salary will be increasing over time! That's why it's important that your retirement savings be started as soon as possible and increase over time.
You'll need to invest your early savings fairly aggressively so that the money grows fast enough to both account for the higher salary you'll earn later on and make up for that last third.
Over forty years, you should expect those early savings to double in value several times even if they're invested fairly conservatively, but it's always better to be aggressive when you're looking at a long time span; even if you take an early loss, you'll end up with more money in the long run.
And that 20%? You could probably cut that down to 15% and still come out ahead, provided it all goes into retirement plans; buying a house, etc, needs to come out of other funds.
It's not a bad idea to start saving for the house as soon as possible; it's nice when you can buy one in cash!