Inventory report and how long since a device reported

Since you mentioned auto.lm.last_check_time not being specific enough, and overall it sounds like you have a specific definition of "available" so I would suggest creating a simple DataSource that will periodically run and check for your definition of available, then set a custom property similar to auto.lm.last_check_time.

Then build a Resource Inventory report with that property (and I'd include auto.lm.last_check_time as well so people recognize the time difference between pingable and operable)