From Eurogamer.net:

Ceccarelli then addressed the question of when games will leave the Games Pass service. In short: nothing will leave before November.

“The bigger thing is when games will leave,” he said. “We see a lot of questions and a lot of assumptions … and while there’s not one set amount of time that a game will be in the catalogue, and some titles will leave the catalogue eventually, I want to put some fears at rest because our first games won’t be leaving the catalogue until late November.”