I did this for the first time 2 days ago on two medium-size GH repos and it while was crazy slow, it eventually completed after about 8 hours or so. The actual bits seemed to finish in a relatively quick amount of time, but the project landing page continually showed "import in progress" for all those 8 hours.
Git is simple to import, but all other related data may be subject to API rate limits.
I've opened an issue with some improvements suggestions, like surfacing when we are rate limited and allowing gradual access to the data while the import is going on: https://gitlab.com/gitlab-org/gitlab-ce/issues/66525