Dashboards are refreshed continuously by a process called the precacher (see my reply here). How frequently a dashboard is refreshed depends on the size of the cluster and the number of dashboards to be cached. For many clusters, dashboards will be cached multiple times a day.
You can determine if the dashboard is using cached data by exploring any of the queries pinned to the dashboard. If the data is cached, there will be a notice like this:
Another indicator of whether a dashboard is cached is if progress bars appear on each chart when you load the dashboard. These progress bars indicate that the queries are being executed in real time.
You can force an entire dashboard to refresh by clicking the chevron in the top right of the dashboard and hitting the GO button:
Users managing their own clusters with Growth Edition (Interana 2.24) will be able to adjust the dashboard cache (using the dashboardCacheDuration.default configuration setting).