How Often Should A Computer Show As Scanned?

I often use the Last Scanned date as an indication of when a computer was last online. We have over 1500 total devices. Approximately 200 of these have dates older than November 1, some back to September.

I left most of the agent settings as default when preparing it so no odd scheduling conflicts. Based on this, what percentage of my computers should display a Last Scan date within 7 days of now?

Thanks,

Sheldon