I know this question is old but I had a similar question and wasn't able to find an answer. This search will find dashboards that haven't been accessed in more than 60 days.
| rest /servicesNS/-/-/data/ui/views splunk_server=*
| search eai:acl.app!="search" AND eai:acl.app!="splunk_management_console" AND eai:acl.app!="splunk_monitoring_console" AND isDashboard=1
| rename eai:acl.app AS app
| fields title app author
| join type=left title
[| search index=_internal sourcetype=splunk_web_access host=* user=*
| rex field=uri_path ".*/(?<title>[^/]*)$"
| stats latest(_time) as Time latest(user) as user by title]
| where isnotnull(Time)
| eval Now=now()
| eval days=round((Now-Time)/86400,2)
| sort - days
| convert ctime(Time)
| table app title days Time user author
| fields - Now
| search days > 60
| rename days AS "Days since last accessed"
Run it over the last 90 days.
If your log retention is set at the default 30 days, look at pgreer's answer above.
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