Do you have any sort of pension?
Will you be eligible for SS? If so, how much of your expenses will that cover?
Historically if you looked at your portfolio at the time you retired and you took out 4% of that portfolio and then each year inflation adjusted that 4% you would have a safe withdrawal rate to last for a 30 year retirement.
So, someone with a $1,000,000 portfolio could withdraw $40,000 in year 1 and then that $40,000 would be inflation adjusted each year after that.
Now, let's say you have $40,000 a year in expenses, but you have $500,000 in a portfolio. If you were receiving $20k a year in SS then you might feel $500k in the portfolio was enough.
Obviously there are many variables that affect this. You need to know what your after retirement expenses will be. Some will go down (don't have to save for retirement, lower commuting costs for many, lower taxes for many, and so on), while others may go up (health insurance and health care).
Also, in the current environment some people feel that 4% is too aggressive. Further 4% is based upon someone with a reasonable mix of equities and bonds. Someone who doesn't have any equities would need a much, much lower withdrawal rate.
Also, how long will your retirement be? Someone who retires early may need to plan for longer than 30 years.