In the latest Technology Preview [0] it's already at 99%. Next release on 25th May will have 100%.

[0] https://developer.apple.com/safari/technology-preview/

Probably at WWDC, between June 13 and June 17.

Likely in the developer previews that they release during WWDC then available to consumers in the fall.

I don't think that it will take this long. They pushed out Safari 9.1 this year, which saw pretty significant changes. I think they could release the things from the tech preview with a 9.2 release for El Cap, maybe a month from now (WWDC coincidently?)