Rule #1 of migrations: "Don't migrate crap".
SuperUser has enough of a problem with people asking low-quality questions that need to be closed or are duplicates of existing questions. Most of the tech-support questions on this site fall into one or the other of those groups, so there's no point in migrating them rather than closing them ourselves.
In the rare event that a question does get migrated, the migration will happen as soon as the question is placed on hold. A copy of the question will stick around on HR for 30 days to act as a pointer to the new location, before being automatically deleted.