You will need a lot, because you never know what the value will be after a few years. The value can be very low and that will be not good.
But the value can also be very high and that is very good. But you life will depend on Bitcoin and that will be not good.
Its always good to invest your money in multiple products, because if you depend only on bitcoins, and if it fails, then you loose everything, and you don't have an option to invest after 20 years.
Investing on different companies to grow you bitcoin is essential.
You will be needing maybe about 300-500 bitcoin with that amount i think you will live a happy life.