If the price doubles every year, the price could be $6000 in 2020. With 100 bitcoin, you will be worth $0.6 million.
Doubling every year is hoping too much.
Somebody offering those returns would be called a ponzi operator.
The price will not double every year ,
the doubling of price of bitcoin occurs only in the halving and after that a slight and regular increase will occur in the price of bitcoin with the passage of time, so with 100 BTC we can not be millionaire in quick time.
The last halving was in November 2012.
Between September 2011 and September 2012 price (BTC/USD on Bitstamp) changed by +105% (i.e. it doubled).
Between September 2012 and September 2013 price changed by +1040%.
Between September 2013 and September 2014 price changed by +240%.
Between September 2014 and September 2015 price changed by -46%.
On average, price changed annually in that 4-year period by +335%, but that's not the point - the point is that doubling (or even significant price increases) has not been restricted to halving years.