You mean, you call Get-Stat and it never comes back ?
Does it make a difference if you only go back 1 week/1 day ?
From where do you run the script ? From the PowerCLI prompt ? Or from an editor ?
Do you see that metric for that cluster for the past month, under the Performance tab in the vSphere client ?
Depending on the size of the vCenter DB fetching the statistical data might take a bit longer.