I was curious about the last reveal so I looked it up.
It happened at the end of October 2014, and the films that were already known at that point were Age of Ultron and Ant-Man (both 2015).
Announced were:
Captain America: Civil War (6th May 2016)
Doctor Strange (4th November 2016)
Guardians of the Galaxy v2 (5th May 2017)
Thor: Ragnarok (28th July 2017 - was actually released on 3rd November 2017)
Black Panther (3rd November 2017 - was actually released on 16th February 2018)
Avengers: Infinity War Part 1 (4th May 2018 - was actually released a week earlier on 27th April 2018)
Captain Marvel (6th July 2018 - was actually released on 8th March 2019)
Inhumans (2nd November 2018 - we all know what happened to that)
Avengers: Infinity War Part 2 (3rd May 2019 - was actually released a week earlier on 26th April 2019 as Avengers: Endgame)
The ones that were not mentioned were the Spider-Man films for obvious reasons (Homecoming came between GotG2 and Thor 3, taking the July 2017 slot), and Ant-Man and the Wasp, which came out on 6th July 2018, the original slot for Captain Marvel.
According to this page here (pdf at the bottom), the future Marvel releases are on:
1st May 2020
6th November 2020
12th February 2021
7th May 2021
5th November 2021
18th February 2022
6th May 2022
29th July 2022
This is all after the Fox stuff so I don't think there will be a Spider-Man style disruption. So a minimum of 8 films to be announced. Last time they did this the furthest planned release date was 4 and a half years. I feel like F4 and X-Men would be around the 2023 stage if not further.
Just to note, James Gunn's Suicide Squad is set to be released on 6th August 2021, so I'm going to guess that GotG3 won't come out until 2023 at the earliest.
Not too long to go now!