PostgreSQL is a non-commercial, all volunteer, free software project, and as such there is no formal list of feature requirements required for development. We really do follow the mantra of letting developers scratch their own itches.
The PostgreSQL project aims to make at least one minor release every quarter, on a predefined schedule. If it becomes necessary due to an important bugfix or security issue, more releases will be made between these dates, so this list should be considered a minimum. At each of these dates, a new minor release will be made for each supported version.
The target date for these releases are, unless otherwise stated, the second Thursday of February, May, August, and November. The current schedule for upcoming releases is:
The next major release of PostgreSQL is planned to be the 10 release. A tentative schedule for this version has a release in the third quarter of 2017.
While there are no formal requirements for each PostgreSQL release, there are several places you can look to find out more information on upcoming features: