How long can we stay on Magento (EE) 1x before we would absolutely have to move to Magento (EE) 2x? I can't find a definite answer to this if there is one. I did read that security patches & support for Magento (EE) 1x will end the 4th Quarter of 2018. Perhaps that is the deadline date everyone is using or hearing? I'd imagine that PCI scanning vendors could start showing hesitance to approving applications that are no longer receiving security updates. Huge concern!!! If so, that would make the 4th Quarter of 2018 everyones deadline to upgrade for sure. I'd rather stay on Magento (EE) 1x for another 5 years but I don't think it's possible. Any definitive answers out there?
I haven't heard a definitive answer but at Imagine 2016, they hinted at 3 years of support from 2.x launch for the 1.x line. That is right in-line with the Q4 2018 date. It seems like Magento is suggesting you migrate to the 2.x line sooner rather than later, but they will support it through 2018.
As for me, and where I work, we have so many custom pieces that migrating isn't going to be easy, or pretty. We're putting it off as much as we can, but my plan is to start shifting the core of my time to Magento 2 migration and less on Magento 1.x. I figure it will take me about a year to convert everything, which leaves us up to a year to test data migration and features.
November 17, 2018 (three years from the date Magento 2 launched) is the last day that Magento 1.x will be supported.
I'm sure that the sales team will be pushing to begin migrations much sooner than that, and if history is any indication there will be those who lag in their builds on 2.x or decide to go it alone at which point they will no longer be supported.
Now. Support and sales are two different things. I'm not sure if there is language about being able to use a version of Magento Enterprise in production outside of the "support" date but my guess is that there will be significant pressure on them to migrate. Market forces like demand and competition; non-market forces like feature and extension compatibility.
All in all you still have a good deal of runway, but as they say: time flies like a kite (and fruit flies like a banana).