I need to rebuild the forum archive. I do this once a year to maintain the forum's snappy performance.
This is when I move old topics to the archive so the active set of topics is smaller and thus faster for the database.
I try to start around 9am US/Eastern time. It takes about 45 minutes and it's best to shut the site down until it's done.
If things go well, I think it could be done by 10am, or noon if something goes wrong. Also, we're on a new (cloud) server now, so maybe the timing will be different, but I doubt it.
I just did the yearly offer/trade message archiving. That can be done while the site is up. It only takes a minute.