Each new version of
jenkins.war is released by the process described here.
This means there's generally a maximum of one week before a change in the master branch becomes available in a release.
There is another release line that lags behind in terms of new features, but provides more stability and a slower update cycle, called the LTS Release Line.
In March 2015, it was decided to make releases directly from the master branch.
Prior to this, there was an release candidate branch which would be branched from master up to a week in advance of the release. But very few people actively tested the RC builds, and this branching model made it more complicated to release a new Jenkins version on demand, e.g. if a hot fix was required.