The $550 7970 was the last time AMD had the upper hand for an extended period, which lasted three months (only speaking of recent generations). Since then, the 780 Ti came out two weeks after the 290x and reigned for ten months; followed by the 980 unrivaled for 8 months until the 980 Ti was released, two weeks before the 390x and three weeks before the FuryX. This, of course, lead to continued unrivaled dominance for one year... until next week when the 1080 is released. On each occasion, Nvidia had a response to every major release from AMD within 2-3 weeks, and the majority of the time was preemptive spoiling AMDs parade.
Based on that history, I would expect the GTX 1080 Ti to come out within three weeks either before or after AMDs top Vega card, most likely before Vega. Nvidia seems to like to hold their secret weapon in reserve and only release it as a means of undercutting AMDs bid for the top spot.