Here's a plan from Dave Ramseys book "Total Money Makeover". You need to estimate how much you want to live on yearly in retirement. Take that number and divide by .08. This gives you the nest egg you will need. This is based on your money being in growth stock mutual funds averaging 12% per year and you living off 8% of that nest egg per year.
If I want to live on $70,000 per year I need to save $875,000 in order to never run out of money. If I only need $50,000 per year then I only need to save $625,000.
Like most carpet cleaners, I pray for a large inheritance because my chances of saving that much are slim.