Six and a half years between the D300s and the D500. Nikon walked away from the market and then reconsidered. Most reports indicate that both Nikon and Canon are walking away from the market this time. However, the D500 also shows that companies do change their mind sometimes. Unlikely, but maybe once all the mirrorless craziness settles down, Canon will see that a niche still exists for a high quality, high performance APS-C body.
Or, they may decide to just keep improving the 90D and in a few years, it will have all the features of a 7DII with a newer generation sensor, flip screen and 1Dx III style thumb control.
Who knows?