Here is a ‘quick and dirty’ tool that I found in Money Magazine (Feb 2011), that gives you a quick assessment to see if your retirement savings are on track.
For each age there is a savings factor (e.g. at age 30 the savings factor is 0.3). This means that if you want to retire at age 65 you need to have 30% of your salary saved by age 30. If you earn $100,000 this means that you would need to have $30,000 saved for retirement. At age 50 the factor is 4.5 which means that if you earn $100,000 you would need to have $450,000 in retirement savings.
Age Savings Factor
This table is a better estimate for younger ages when retirement is far off and you want a quick reality check. As you get closer to retirement so many other specific factors could affect your number (e.g. will you have a mortgage, will you move, will your lifestyle change, etc.) that it may be worth doing a more specific sophisticated analysis, or even seeing a Fee-Only(tm) Financial Planner.