The only time I remember HubPages being down for any length of time was in December of 2006 a few months after I joined. As I recall, it was down for about an hour and there was no message other than a "server not found" that appeared when I attempted to access it.
Since then I think there have been three or four occasions when I have received a message to the effect that HubPages was down for maintenance. On these occasions HubPages was either off for a very short time (minutes) or I happened to access HubPages a few minutes before the team finished and put the site back online.
However, these few times I have encountered the site being down has at most been a very minor and short inconvenience.