You can use some sql statements when creating a query to make them more dynamic. One example is to create a query showing machines that haven't scanned in for more than x days.
So choose Computer item on the left side and Last Updated by Inventory server on the right side.
Next choose the < in the middle field. Finally, type the following into the field that lets you specify a value: GetDate()-30
Add this to the query and it should look like:
"Computer"."Last Updated by Inventory Server" <"GetDate()-30"
Note: For Oracle databases, replace GetDate() with SYSDATE.
Save the query and each time that you run it you will computers that haven't scanned in for more than 30 days.
NOTE: Here is an article with screenshots that will also show how to create a report from the data and send the report in an e-mail: