The PX4 project is currently under a 6mo release cadence; the PX4 1.9 release has a target release date of January 2019.
I'm proposing the following timeline
July 2018 - November 2018
On-going development work for PX4 1.9, and minor releases for PX4 1.8
November 2018 - December 2018
No new features merged into the 1.9 release. The focus will be on hardening the features that made it in. This is the cutout phase for any feature, features not ready will be pushed to the next release.
Code freeze - Flight testing
December 2018 - January 2019
Only security and bug fixes make it into the codebase, this is a period where the Flight Test team has a heavier rotation cycle on tests, this is the window where we guarantee the safety of the release
Due diligence for the release, release notes, contributor acknowledgments, we publish the final artifacts on Github.