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 between 2%-5%. The percentage of currently Bitcoin users in the world is lower than 1% I think. In 3 years it won't be very different, it's a short time period for big changes... I'm being optimistic to think it can reach to 2%-5%, it only will happen if the Blockchain be improved, transactions delays issue be fixed and a nice marketing be done to bring more users.