How long will Ultegra 6800 parts be available? With the recent release of the new Ultegra 8000 groupset, is there an official word on how long Ultegra 6800 parts will be available?

Is Shimano still manufacturing Ultegra 6800 parts?  Or have they stopped producing 6800 and it's only a matter of how long resellers inventories will last?

Is there a way to somewhat reliably estimate the likelihood that, for example, an Ultegra 6800 50/34 170mm chainrings will be available 6 months from now?  A year from now?  2 Years from now?

For those with substantial past experience, with the release of a new Shimano groupset, how long do resellers still usually offer the previous group for?
Don't expect something official. If you don't see it listed on the main page of Shimano's website, production has likely ramped down or stopped.

Resellers will go as long as theres demand and stock. This also controls availability, and its tricky to estimate length of time, especially on less commodity groups (e.g. Ultegra vs Sora; you'll have a lot more Sora spare stock around, just cause a lot more people have Sora). Some retailers show stock levels on their websites, which could give you a rough idea. 

This will also depend on your regionality and other things too, so don't expect something reliable. 



That being said, you shouldn't worry if you want to just keep things working at a comparable level -- innovations from higher group sets do trickle down to lower ones. So, next year's 105 chainrings may be essentially identical to your Ultegra rings, for example. You could also mix with the new Ultegra (or other sets) where possible. It isn't the end of the world to not have a full set of one kind -- you'll be able to find parts in other group sets or manufacturers which are compatible (especially for things like chainrings). And if you really want a full set of one kind, well, you might end up ponying up for the new one in the future if its that important to you!

Of course, there are exceptions to the prior statement, when Shimano introduces a big incompatibility (e.g. mixing pre-97 ish dura ace with anything requires some trickery, and the Dura ace 7800 had extra tall splines making some compatibility issues for people; price of being on the edge, I suppose0.