There's no one-size-fits-all approach to saving for retirement. If you want to retire earlier than the traditional retirement age, this "Financial Freedom Fund" calculator will tell you how much to set aside.
Robert Berger explains the math behind it on Forbes:
The math behind the Financial Freedom Fund is a simple two-step process. First, we need to calculate how much we need in investments to generate sufficient income to fund our living expenses. Second, we calculate how long it will take to reach this goal based on our current savings rate and an assumed rate of return. Once we have this information, we can then adjust our savings rate based on when we want to retire.
The spreadsheet offers scenarios for rates of returns from 5% to 9% and savings rates of 5% to 95% (in 5% increments). So when viewing the spreadsheet, you can see that if you want to retire in about 30 years and your investments return an average of 5%, you'll need to put away 25% of your income. Use this link to make a copy of the Google spreadsheet to edit yourself.
The spreadsheet assumes a 4% withdrawal rate in retirement and that you'll want to completely replace your income in retirement. You might very well live on less when you retire, but it doesn't hurt to oversave and at the bottom of the spreadsheet you can enter different numbers for your income in retirement, withdrawal rate, and investment return.
I like that this calculator takes into account your current savings and shows at a glance how much earlier you can retire if you ramp up your savings a little more or if your investments earn just one percent more.