It depends on a number of factors. The main question is how much interest do you need? You can then use a financial calculator to back into the amount of money needs based on expected return. The expected return is based on the investment vehicle your money is in.
In US Dollars, I believe the ideal amount would be 6 million as long as you have a way to keep 6-10% interest coming in at all times. This level of interest would address inflation issues.
You need to calculate how much interest income you need to live ( say 50,000 ) and find the highest safe interest rate you can find and calculate principal needed.
Three variables here: the principal (the amount in the bank), the interest (money earned), and desired income level. The higher the interest rate, the lower the principal needed.
