Im going to have to go with a conservative estimate of 5% of the worlds population will own bitcoin by 2020. 5% of the entire world population is enough people to create a country that has more people than the USA. There will definitely be more widespread usage of bitcoin as a payment method, and more governments will have to approve of bitcoin.
I think 5% for the estimated of the user of bitcoin for 2020 is very low because i think it is more than 10% now and it will keep growing because bitcoin is unstoppable and its price is keep on rising. The 3 years left will be enough for bitcoin to climb at least 20% of our population because we can see a lot of information that are keep spreading thru the internet and that will be a big help for bitcoin.