We have solved this via two functions that are in place in LS.
When i rent out a asset to User "A" change the state to "Rented out" (We use a custom asset state ) and then add a comment , eventually you could also add a relationsship between the asset and the user.
Then we have a report wich reports asset witch has state "Rented Out" and no comment the last X days. The report sends alert each day (if it has any assets ) and so we get a notification when the user has rented a asset for more than X days.
(We use a custom widget to automate all the steps in this process)
