There is not a specific canned report that shows this. However, you can use the "Events" report to show information similar to what you're looking for. In the OnCommand Console, using the Events tab, you can customize the colums to show you all events for all severities. Then Filter the event column on "Host*". You can see all the Host Down and Host Up events, and from that determine how long a particulr host was down.
Note: this information is stricly from the perspective of the DFM server, not from the storage array itself or from the application that uses the storage. It is the user's job to confirm if the reported downtime was actually an outage, or just a network fluke. For example, perhaps the DFM server lost network conectivity to a storage array that host FCP LUNs on a SAN fabric. The SAN fabric was available and data was being served to SAN clients during the network outage, but DFM would report the host as being down until network connectivity was restored. In this scenario, this clearly would not be categorized as downtime.
Good shout with filtering on the host up and down events, not finding many matching host down and host up events,do you know why that would be? I was expecting a one to one correlation. I am assuming that some of the events are being deleted by another DFM user/admin?