You can use the Auto-Refresh option in the widget configuration page.

It will refresh the widget state and check if there are any new announcements to display in widgets and boosters.

Note: the Auto-Refresh option is available on version 1 (classic) widgets. Version 2 widgets don't poll for new posts on a timer — they fetch the latest state when the page loads and when the widget is opened. If you need timed refreshes on a long-lived page, you can keep using a v1 widget or trigger a re-render from your own code.