Expect it later this year. As has been mentioned multiple times, PC got early flights because there’s a lot more development required. Additionally, MCC as a whole is going to be a new title for PC, so the Reach flights serve to test the overall game too, I believe. Don’t worry, Xbox has not been forgotten in the slightest.
None of these answers are really helpful for OP. There's no set date, 343 will announce it when they have a date in mind. Patience is a virtue. They just want to build hype. It's a good marketing plan.
Currently the PC build is the focus point of the first flights. The most likely reason for this, is that Reach on Xbox is a much simpler task in comparison and the PC version requiring thorough testing, to gather as much data as possible. They want to avoid another MCC launch disaster afterall.
The next flight (for PC so far) is gonna be firefight, most likely followed by a Multiplayer flight a couple of weeks after. In short, we'll most likely get an Xbox flight in about a months time, rough estimation based on technical babble and what not.