Halo: The Master Chief Collection is receiving a staggered release for PC, although we don't have a time frame for when all the games will launch. However, according to co-developer Splash Damage, the entire collection is going to come out by the end of the year.
The newsletter (via ResetEra) states clearly that 'Halo: Reach is coming first, with the rest of the titles scheduled throughout the year,' seemingly confirming that all of the games – Halo: Combat Evolved Anniversary, Halo 2 Anniversary, Halo 3, Halo 3: ODST, and Halo 4 – will be out in 2019.
One thing that you probably shouldn't expect from Halo: The Master Chief Collection is Play Anywhere functionality. However, you can gain early access to the game thanks to the arrival of Halo Insider on PC, so that's handy.
Halo: The Master Chief Collection was originally released for Xbox One back in autumn 2014, and developer 343 Industries has continued to support the title ever since.