When is Shimano Di3 going to be released?

Sorry, it doesn't work that way. Di2 stands for Digital Integrated Intelligence. Of course Shimano could add another I to that and call it Digital Integrated Intelligence Improved (Di3)… but it’s more likely that they will just increase the existing model / product codes.

Having said that though, Shimano are rumored to announce and release Dura-Ace 9200 sometime early 2021. When they do, it will probably use the newer "next-gen" EW-SD300 E-Tube wires instead of the current EW-SD50.

These newer wires are fully compatible with current Di2 components, but the plug itself is different. An adapter exists though, so you should still be able to use the components you already own (think shifters, switches)