The 4 % Rule
How much money do you need to pile up to have enough passive income to stop working?
If you invest into the average stock market, there is an answer. Figure out how much you want to spend per year and multiply it by 25, because you can withdraw 4 % for eternity. Read the full explanation in this Mr. MoneyMustache Blog article.
BTW - If you think this is a silly question in a blog dedicated to work, let me tell you it's not silly at all. Money is nothing else but a storage medium for work.