Let's say, the retirement age is 66. Good health. Let's assume life expectancy at 88 years.
This leaves you with 22 years before you expire. Let' say average lifestyle is about 10K/month. Let's say you were making good $$$ while working, now your social security at 66 is about 3K.
So, you need that extra 7K/mo for 22 years. 22 years is 264 months. So, 264 months * $7K rounds up to about $1.8M.
I hope you have that $1.8M saved in your 401K.
How is my math?