How to view last updated date for report?

Depending on where your .pbix files are stores, you may be able to read the last modified date from the host.  If the files are stored in unpredictable locations you might need to store the last edit date inside the model manually (see: https://powerbi.microsoft.com/en-us/documentation/powerbi-desktop-enter-data-directly-into-desktop/ ) and ask report authors to make it a practice to update it manually after every change.  This might be a pain, but it might also creates the opportunity to summarize latest changes or add addition value to your users as to what has changed as well as when the report has changed.  By storing the meta-data inside the model, or even as another datasource in some centralized location, it's easy to display in reports and dashboards.