My take is that browser support will be very fast, because of evergreen browsers. I'm also guessing a majority of the top 500 sites will support it pretty quic

My take is that browser support will be very fast, because of evergreen browsers. I'm also guessing a majority of the top 500 sites will support it pretty quickly (on the order of a year or two). Everything else, will be a long tail of a decade or more. Edit: Just wanted to add that, I think the better goal is "majority of traffic using HTTP/2" - that's probably 5 years because the majority of traffic probably comes down to the top 500 sites.