Early retirement

Question: How much money do you need to retire around 40 years old?
Answer: It depends on how you invest, how many years you are going to live, bla, bla, bla. But as an engineer I´m used to estimate. If you ask me how long it takes to fill a bucket with water, I won´t answer that it depends on the amount of water per second or the size of the bucket..., but around 1 minute. So, to answer the first question, after statistic testing, the percentage of money we must use is 4% a year of the total amount.

Example: I need 4000 euros a month. 4% x Total Amount = 4000 x 12
Total Amount = 1.2 Millions
When I get 1.2 M, I might retire.

Nice link: Retire Early.