This depends on the lifestyle you want to live as well as your work history (for Social Security purposes) plus (to some extent) your family history. Obviously, if you think you can live on $40,000 a year (maybe some weekend trips, little eating out, etc.) you will need less than if you plan on a more "extravagant lifestyle (one tip a year, eating out 2 or 3 times a week, new cars every now and then). If your family history suggests you will live a long life (never guaranteed but genetics should be taken into account), you will need more money,
A good rule of thumb is that you should plan on spending 4% of your retirement funds each year (adjusting for inflation). If you can live on $50,000 a year, you will need at least $1,250,000 in your account. Given you want to retire early, probably more (closer to $2 million) is needed. You also need to have this money invested in a somewhat aggressive investment (not all bonds, for example, but not all stocks - possibly a 50/50 split) which will continue to grow the accounts.