I just hope it doesn't take 20 years the way it took the internet that long to get to where it is now. (i.e. from 1994 / Netscape - Thanks Mark)
Expect widespread global mainstream adoption within 2-5 years. The Bitcoin protocol will continue evolving forever, just as the internet and its related technologies are still evolving today.
Think of the blockchain like the foundation, just like HTML is the foundation for all the internet that was built on top of it. Any software engineer will tell you, no software is ever "complete".