It depends on the standard of living that you consider satisfactory, and the country where you will live, since the cost of living is an important factor. But I believe that anywhere, 100 BTCs would be enough in twenty years.
Well, if you will just live within your means then 100 BTC is even more than enough to enjoy life, if I will be ask I would also make the same answer because I do not want to live a lavish life to be happy, I just want my life to be simple and do a healthy living with my family.