1.We may get the cases saying that How to set auto refresh interval for dashboard or how long will the dashboard refresh by default?

2.As of now, We don't have the Auto-refresh functionality for dashboards.

3.Please go with manual refresh option using "Refresh" button as given below.

Please find below community page where experts say "This refresh option is not available OOB as it does not suit with the dashboard architecuture."

https://community.servicenow.com/community?id=community_question&sys_id=f8823f38db64489414d6fb24399619e1