Sorry but i can't see AMD just going bang and taking 50% straight away, I think it's the other way around Intel & Nvidia still have plenty of tricks up there sleeves.
Remember it's AMD that has to come out with a game changing CPU & GPU, Intel & Nvidia doesn't well at least not at the moment anyway.
IMO Intel & Nvidia are just sitting back and waiting for AMD and then they will go from there. Which there allow to do.
No, I mean a few years from now, maybe more. I don't think Vega will blow away the competition and retake 50% of the market. Even if Vega is miles ahead of nVidia, it's still considered out of the price range of most of the market so will only serve as filling a niche void (at least in gaming) and pushing the name of AMD further.
What I mean is this:
AMD releases an excellent platform like Zen; Intel responds with an equal (Kaby Lake).
AMD refreshes Zen; Intel responds with the superior Cannon Lake.
AMD introduces a new range that performs better than even they expected; Intel responds with an on par product.
AMD releases a dub; Intel refreshes their range.
AMD releases a brand-new architecture that completely blows even their own preconceptions out the water; Intel is left lagging.
AMD releases a refresh; Intel responds with a stronger new architecture and beats AMD again.
In that 2-6-year timeline, Intel is consistently one step ahead—except for that time when AMD fell onto something brilliant at the same time Intel fell onto an unpredictable hardship in their fabrication. That 1-2 year window, along with all the years prior to it, brings AMD back up to, at least temporarily, a 50% market share. It then drops back to 40-45% once Intel rearrange things and beats AMD again. And around the same time, nVidia only matches AMD in the GPU race.
Of course all this is conjecture and hypothetical, but you understand what I mean. It's a slow process, and a lot of it is purely good timing.