AutoPilot provides many derived metrics about the facts being published. In this case, you would want to use Update-Age and Change-Age. The values are in milliseconds and represent the time since the fact was last updated or last changed. The difference being that Update-age is reset to 0 when any value is published and Change-Age is only reset to 0 when the actual value is different than the previous value.
For example, testing f0:Update-Age is greater than 30000 would indicate the value had not been updated in more than 30 seconds.
However, to optimize performance, AutoPilot only drives sensor logic when the specified facts are published. This means that if the sensor only subscribes to this one fact AND if the expert stopped publishing it, the logic to test the Update-Age would not be driven. To be driven periodically, the solution is to include any fact that changes regularly to the facts included in the expert. One example is under System facts, the CEP System fact System/time value is updated every 30 seconds. This fact does not need to be used in the sensor logic, simply selected as a Fact to Monitor.
As an example:
When %f0:Update-Age is greater than 60000