I think around 3% because there are a lot of people using bitcoin already but there are also a lot of fake accounts from the same person we cannot see trough them all so yes but i still don't think that in 2 years bitcoin can go higher than 3%.
It can be measured by how many addresses are generated, and because an individual can use more than one address, this is much harder to count unless some statistics groups conduct a legit survey. This currency is worldwide, it can't be count so easily. They can only use historical datas to speculate what is the percentage of people that might be using BTC in 2020 and beyond. I have watched some pretty informational video (cant remember where) regarding this topic and regarding the effect of the number of users to BTC's price. That was just pure mathematical computations but worth considering for.
The services that can be paid by BTC must increased in order for the demand to be bloomed and in order for many people know about this currency and its edge to other traditional currencies thus increasing its users and owners.