The major goals of stable releases are:
Reminding people the project is still around, because some people don't seem to get the 'rolling release model' where new features appear continuously.
Providing a target point for translations and tilesets, it's incredibly hard to hit a moving target like our daily releases with translations and tileset additions, so the stable releases give us a point where we can sync them up.
Having an official bugfixing period before a release to try and nail down some bugs that might otherwise linger for a long time.
The actual time period is a function of the above goals, currently we're aiming for once every two months, but that's very likely to slip on a regular basis, and we're not too bothered if it does. I'm planning on marking a number of issues as release blockers for 0.C as soon as I get back from the holidays, which is the first step in the release process.