Since when the VIPs/Nodes are offline

If you click on the pool member in the pool overview it will list the Availability which will say something about since when that is down. If there is a relation upto the the virtual server you also know since when that is down.

If you are sending the logs to a syslog (or such) server you can look for the virtual server / pool up / down messages. On the box it will be only like 7 days.