It's gonna be awhile. Probably about a month before the actual release to everyone. There are to many bugs and they don't want to release to everyone until it's all but final. I opted in more than 3 weeks ago and still haven't been invited to the program. Right now only people with high feedback are going to get in and since I have never posted feedback in the preview program, I will probably be last on the list.

Most likely going to be a few weeks. I'm quite high up on the list (4,000 global leaderboards) but I have a friend that had 200 feedback points and they got in.

It seems like there have been new waves of invites going out about every other day or so. Lots of my friends that didn't get it last week have been getting it these past few days. I doubt this will last long though, invites will slow down as they near capacity. All you can do really is wait and hope you get picked to be a tester.