100bitcoins and a well settled busness will be more than enough to get retired after 20years but i think 100bitcoin is little bit harder to achive in this time but you can try
It may be more than enough, but it depends on how the currency will rise in value over the years.
Depend on his need to wag money.so you can not say 100 bitcoin will be enough .if you looking to retire after 20 year than why not to invest 1 btc in trustable mining service which give you good profit.
Things and foremost theres no a trustable mining service even they exist for how many months now but the assurance that it would last forever isnt there. 100 bitcoin is a good amount if you looking to retire after 20 years. You could build a physical business with that amount and you can secure your future i guess.