Only Adobe knows, and they don't tell.
All you can do is wait.
The good news is that Nikon updated their SDK on 2016-05-25.
The earliest I would expect LR to include it would be 1 1/2 updates after that date (the half due to misaligned release schedules).
Lightroom CC (2015.7) and Lightroom 6.7 are now available and it includes support for tethering Nikon D500.
Please update Lightroom to latest version from Help>>Updates menu.
Release notes:-Lightroom CC 2015.7 now available