You never merge 'next' to 'master'. You merge topic branches when the topic is considered to be complete and stable (it "graduates"). The rerere [1,2,3] feature (a fantastic set-and-forget feature) ensures that you won't have to resolve the same conflict multiple times.
The amount of time required for a topic to stabilize in 'next' depends on the topic and what it affects, but you can easily summarize "branches in next, but not in master" to look for candidates.
Feature releases are tagged on 'master' and 'next' is usually rewound at a release (create a new 'next' branch starting at the release, merge all the branches that failed to graduate in this release cycle, and discard the old 'next'). This is easy to automate.
The amount of time required for a topic to stabilize in 'next' depends on the topic and what it affects, but you can easily summarize "branches in next, but not in master" to look for candidates.
Feature releases are tagged on 'master' and 'next' is usually rewound at a release (create a new 'next' branch starting at the release, merge all the branches that failed to graduate in this release cycle, and discard the old 'next'). This is easy to automate.
[1] http://git-scm.com/2010/03/08/rerere.html
[2] http://www.kernel.org/pub/software/scm/git/docs/git-rerere.h...
[3] http://gitster.livejournal.com/41795.html