That question drives me nuts. There is only one answer; that depends. It depends on your lifestyle and how much you earn because a major factor for most people is their Social Security benefit.
So, how do YOU figure your answer. You can do this. The tools are readily available.
Following is a very general example using SSA tools and a Future Value Calculator available to anyone.
Let’s assume you earn $50,000 (or will earn that at retirement).
Go to the Social Security calculator and enter your information. Your monthly benefit at age 66 in 2019 is $1,397 or $16,764 per year. NOTE: You can use a SS calculator that is based on your specific earnings record with the SSA and will project future inflation adjusted dollars.
Assuming a conservative (my idea of normal) 100% income replacement, after SS you need to come up with $33,236 per year. You may be happy with 90% or even 80%.
Using the simple 4% withdrawal rule, you need $830,900 on the day you retire. Now, how do you reach that goal?
If you are saving for thirty years and earn only 6% a year on investments, you need to save roughly $800 a month (Better converted to a steady percentage of your growing income).
There is a great deal of flexibility here. For example, load up on savings in the early years and the task is easier. If you have an employer match on 401k savings, your burden is less.
You may earn more than 6%. Adjusted for inflation, the historical average annual return for the S&P 500 is around 7% and is substantially higher if all dividends were reinvested which you certainly would do.
Of course, we are talking long-term, not every given period of years will reach those earnings or any earnings for that matter.