For released stuff, it only needs to be built once the release is ... released.
For HEAD version, it should be rebuilt every night.
I haven't bothered too much with the code to building the repository yet, since we can always improve on that after the release is out, it's only going to sit on openacs.org.
The code to build the repository is in acs-admin/www/apm/build-repository.tcl - it's not linked from the UI, since it should generally only be run on openacs.org. If you want to host your own package repository, though, you can.
I've put up a repository for HEAD at http://openacs.org/repository/5-0/manifest.xml.
I haven't configured it to automatically rebuild, so the HEAD packages will stay current as of about 1 hour ago until we release 5.0.