The disk usage plugin could use some redesign. It currently will fire off every 60 minutes regardless of whether any new builds have happened. This is kind of a waste of effort since we really only need to check when a build completes.
There are better ways of enabling disk usage monitoring, for example, BuildWrapper extension lets us get notified whenever a build is setting up and gives the plugin a reference to a BuildListener. This BuildListener will be notified when the job completes. It's at this point that the disk usage should be recalculated..