New posts are imported by WP-Cron for every feed a gallery showed in the last 30 days: a full sync every 2 hours, plus a 5-minute job that refreshes one account whose data is older than 3 hours (1 hour for RSS).
Saving the Social Feeds settings imports new accounts right away. A source you pick in the block is fetched the first time the editor previews it. The Sync Now button on the Social Items screen runs a sync on demand.
A real cron job makes this reliable. See CRON.