Put down an estimate of how long do you think you will live, factor in the same for spouse if you're married/will marry, factor in different kinds of expenses (health, essentials, white goods, vacations, and others that influence lifestyle) and most importantly, if you have/plan to have kids.
After all this, use a long term inflation figure of 8% to see how much money you'll need.
If you're not some big shot born with a silver spoon, you'll most likely drop this idea of early retirement. Not because you can't do it but its awfully difficult to create that kind of corpus at that early an age, unless of course there is illegitimate money involved, you're one of those outliers who draw a crore-range income at a young age, or as said above, you're born rich and have a sizable inheritance to come.