I ran the migrate_package_activity.py (https://github.com/ckan/ckan/pull/3972) on a real existing CKAN database to get a sense of the time taken and the change in database size
Timing: Migration took 49s for 236 datasets. If you scale that linearly to 900k datasets (europeandataportal.eu) then you're talking 1 hour. There's likely to be more activities per dataset though, so it would be more like several hours. Seems reasonable to want to do the migration while the site is up.
Size: size of the activity table went up about 20%, because every revision we now store resources, extras and tags on each package. However the overall size came down when we deleted the content of the activity_detail table (which is no longer needed), because that had a quirk that when you add a tag, it stored the dictized package as well in that table - this seems like a mistake.
Test database properties: