It looks like [0] & [1] are the relevant DB migration scripts. GitLab.com is used to test new release candidate versions in a production environment before it is shipped. I'm guessing the 10-15 minute number came from how long it took them to run it on their systems. They do have staging environments where they run migrations (and test new code) on stale prod data as well.

[0]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/db/migra...

[1]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/db/migra...