The script can be rerun to import data that is new since the previous run. So you don’t need to take the forum down until you do the final import. Scripts that I’ve touched allow you to set an IMPORT_AFTER environment variable that will omit data from the import (otherwise it reads but skips the data).
A couple importers have a bulk importer which speeds things up, but it’s more complex to run.
You’re likely looking at a week or two of runtime alone for the first run.