There are two popular ways to manage software releases: feature driven or timeboxed. In a feature driven project, each version commits to a predetermined set of changes and the release is done when everything has been finished. In a timeboxed project, releases are done based on calendar dates without committing to any particular set of features. The latter method is used for example by modern web browsers and Ubuntu.
We more or less followed the timebox method during stable releases 1.9.10–1.14, having a roughly three month release cycle for each version. However, during the 1.15 unstable builds the schedule was first extended and then continually delayed. It eventually became clear that we had abandoned the notion of timeboxing completely.