To the OP: In asking this question, is it to determine whether the 6D is worth owning for you?
Given Canon's history of introducing new full frame cameras, but taking into account the lower price segment of this new class of full frame cameras...my guess is at least 24 months. If this were 2010 rather than 2013, certainly it would be 36 months or more. It really all depends on what Nikon does, and to a lesser extent, Sony.
The rebel series all sell for $1k and less, and are thus updated more often. The updates they usually get are not major. The rebels are meant to be a compromise, entry level camera. The 6D is meant to be a compromise full frame entry level camera.
The question is, does the market want a "bare bones" autofocus on a full frame camera? Has the Nikon D600 outsold the 6D? I've not seen any official sales figures from Canon or Nikon, and they might not ever release them for specific bodies. The aggressive price drops recently of the 6D, might be an indicator of poor sales, or it might be an indicator that Canon makes an incredibly high profit on this model, no matter how low the price goes.
Certainly the price of used 6D's is holding up, with the lowest on amazon marketplace currently $1624.39. I paid $1760 for mine new in March from Adorama, four and a half months ago!
When the time comes, the 6D's replacement will possibly use the same image sensor, but using "digic 6" or "7". Whether the autofocus sensor gets updated with just minor tweaks, or gets something higher in performance, is probably more due to whether the market wants a $2500 body rather than an under $2k body. Time will tell, they will know what to build before then. Certainly if the 5D3 does not get a major update before then, an amped up 6D definitely would cut into the 5D3's sales...especially if they fix the moire in the video of the 6D.
So it seems to me the 5D3 would get some kind of update before the 6D does. And Canon will not give the 6D line enough bells and whistles to displace the 5D line. 3 to 5 years from now, as the second update cycle approaches, it's possible the 5D and 6D lines would morph together into one....or not. Just depends on what the market wants, and what other models or product lines are introduced before then.
As for using the 6D for professional stills photography, I have found it works great. At its fps, it does not need the faster compact flash media. The files are usually just small enough to be dealt with. Shallow depth of field files (often present in action shots) are often 20MB and smaller, RAW. As jpegs they're obviously even smaller.