Widgets and dashboards refresh independently. Find out how often widgets refresh.
Dashboards have two main components: layout/settings and widgets. This article explains how often these refresh.
Dashboards as a whole automatically refresh every 12 hours. This is to ensure the latest layout is displayed everywhere (TV screens, sharing links, etc) and for everyone (including view-only users accessing the dashboard).
You can force a dashboard to update using the Refresh Remote Dashboards feature. This is particularly useful when you're testing something new or making significant changes to a dashboard e.g. modifying the layout of the widgets, adding or deleting widgets, changing the dashboard theme, etc. and you want to see those changes immediately reflected on your screen or shared link.